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Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


Have you ever wondered how roads are designed? Ever questioned why the traffic signal down the road needs two sets of signal heads? Have you ever stared in disgust at a road and wondered, what kind of idiot designed this, and how does he remember to breathe?



That idiot is me! I'm a traffic engineer, and I design roads for a living. Well, that’s not entirely true… I mostly fix other engineers' 50-year-old mistakes. I've gone through years of schooling in the field, and worked in both the public and private sectors in 2 countries: the USA and France. I'm quite willing to share my insights and industry insider knowledge with those of you bold enough to ask. I'm rather knowledgeable in the fields of traffic (including signs, stripes, and signals), highway design, and urban design.



I'll tell you everything you want to know (and more) about New England's roads, specifically Connecticut's and Rhode Island's, since that's where I've worked. Engineering rules and theories are quite region-specific in some cases. I'll try my best to specify whether the things I'm writing about apply to one state, to the country, or to the whole world. Please correct me if you see something that could be incorrect.



A few suggested lines of inquiry, as I don't want this OP to get too long:
- Why New England's freeway network is so screwed up (Hint: it's your fault)
- How we decide where a road is needed, and how it should be built
- Dealing with the mentally disturbed and the woefully ignorant
- Highway standards in Europe vs. the USA
- A primer on interchange design (so you don't have to read the Green Book yourself)
- How did this happen? (You show me a highway gently caress-up in your area)
- Highway oddities: Spupclos, stop signs on on-ramps, horrible designs that nearly came to be…
- Why mass transit sucks in the USA



One disclaimer, and keep this in mind: My opinions on speed limits or red-light running or anything else of that nature should not be construed as giving you permission to perform illegal acts. Some of our traffic laws may be arbitrary, misguided, and obsolete, but you still have to obey them.

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Crackpipe
Jul 9, 2001



Can you guys go back to paving highways with concrete again? Please? I-84 crumbles every five years because The Powers That Be insist on going with asphalt, yet the concrete sections have been holding up since before I was born.

I know there is often concrete beneath the asphalt, but it isn't helping.

Silver Falcon
Dec 5, 2005

Citizen of Zada

Ooh, this sounds interesting. I'd love to hear about how the highway standards differ between the U.S. and Europe. Infrastructure seems overall better over there and I've never been able to put my finger on why.

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


Crackpipe posted:

Can you guys go back to paving highways with concrete again? Please? I-84 crumbles every five years because The Powers That Be insist on going with asphalt, yet the concrete sections have been holding up since before I was born.

I know there is often concrete beneath the asphalt, but it isn't helping.

Ask as often as you like, but your complaints will fall on deaf ears. We've been using asphalt for so long here that it's very unlikely things will change in the near future.

A big issue with concrete pavement is that it can last 50 years, whereas asphalt is meant to last 7-10 years. A lot changes in 50 years. Maybe your road needs repaving after 10 because you widen a shoulder, and you have to tear up concrete and waste all that investment. Maybe a sewer line breaks and you have to bust up your nice flat concrete.

There's also the issue of ride quality. Theoretically, concrete is rougher to ride on than asphalt. The concrete industry explains that this is because most of our concrete is 30-50 years old, so of course it's rougher than the 5-year-old asphalt that was built to modern specifications.

Concrete paving can also lead to problems at joints if the dowels aren't placed properly. One example is on US 1 in Rhode Island, where riding on the concrete road was something akin to trying to drive over railroad ties at 60 mph. Given the limited money we have to work with, I don't doubt a lot of our roads will be built shoddily. I'd rather be able to fix it in 10 years than in 50.

And finally, this is more of a political thing, but SOME jurisdictions(no names!) purposely build lousy pavement so that their contractors can soak up more maintenance money and stay in business.

Edit: If you think the concrete sections of I-84 are in good shape, assuming you mean the CT portion, take a close look at the Aetna Viaduct, which is currently under emergency repair work. It looks like cheesecloth in some places.

Frinkahedron
Jul 26, 2006

Gobble Gobble


Did you study Civil Engineering?


And to kick off a suggested topic, I would like one of these:

- Highway oddities: Spupclos, stop signs on on-ramps, horrible designs that nearly came to be…


I seem to remember a topic here or in GBS where you posted some of those and they were pretty freaking neat.



fake edit: If you could start the traffic network around the DC area from scratch, how would you do it?

Crackpipe
Jul 9, 2001



Have you planned any busways?

Both L.A. and Boston have run into serious problems with the pavement essentially collapsing after a short while. I think the Orange Line in L.A. is on its 2nd re-pavement or so since 2005, while the roadway of the Silver Line tunnel in Boston is literally crumbling back to gravel.

Is this "normal" given the weight of the buses and the fact they're traveling over the same narrow patch of pavement pretty much constantly, or is it just shoddy construction?

Cichlidae posted:

Ask as often as you like, but your complaints will fall on deaf ears. We've been using asphalt for so long here that it's very unlikely things will change in the near future.



Bah, have it your way traffic engineers!

Cichlidae posted:

Edit: If you think the concrete sections of I-84 are in good shape, assuming you mean the CT portion, take a close look at the Aetna Viaduct, which is currently under emergency repair work. It looks like cheesecloth in some places.

Yeah, I was thinking of the sections immediately west of Manchester which aren't in the best shape, but they're older than several typical goons combined.

Chairon
Aug 13, 2007
I once was a man. Well,I suppose I still am.

I was driving along a highway(99, if you have to know) near Sacramento, CA when the road gets all these lines carved in it, maybe half an inch deep or so, across all four lanes of traffic. They cris-crossed diagonally, very close together. There wasn't any road work going on, and this went on for miles and miles, in both directions. What would be the reason for this? Is it just the design of the road? It's really noisy and I think it'd chew up tires pretty quickly. I also made a horrible Paint to show what I mean! The black is the road, and the blue is supposed to be said lines.

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


Frinkahedron posted:

Did you study Civil Engineering?

And to kick off a suggested topic, I would like one of these:

- Highway oddities: Spupclos, stop signs on on-ramps, horrible designs that nearly came to be…

I seem to remember a topic here or in GBS where you posted some of those and they were pretty freaking neat.

fake edit: If you could start the traffic network around the DC area from scratch, how would you do it?

First off, yes, I've got degrees in Civil and Environmental Engineering, French, and German. That was an interesting curriculum, to say the least.

Highway oddities will take a while, as there's a lot of ground to cover. Look for some hilarious (and frightening) drawings and photographs in the days and weeks to come.

DC is a complicated place, to say the least. Looking at the surrounding cities and pulling some numbers out of my rear end, here's a diagram of how I'd design it. I'm making some assumptions: Baltimore hasn't been reduced to a smoldering crater, I have to preserve the downtown DC area (freeways there are built with tunnels, very expensive), and the airports are full intermodal hubs with train stations for the Metro and commuter rail.



You can see, the basic idea is redundancy, and providing a nice bypass for through traffic on I-95. Most major origin-destination pairs have a direct freeway route.

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


Chairon posted:

I was driving along a highway(99, if you have to know) near Sacramento, CA when the road gets all these lines carved in it, maybe half an inch deep or so, across all four lanes of traffic. They cris-crossed diagonally, very close together. There wasn't any road work going on, and this went on for miles and miles, in both directions. What would be the reason for this? Is it just the design of the road? It's really noisy and I think it'd chew up tires pretty quickly. I also made a horrible Paint to show what I mean! The black is the road, and the blue is supposed to be said lines.



Replying to you first since it's an easy question. That's grooved pavement. Asphalt isn't all laid out at once; we put down the base course, then the leveling course, then the wearing course. Often, days go by before the next course gets laid. If you've ever made pottery before, you'll remember that wet clay doesn't stick to dry clay, and you have to score it first to get more friction. This works the same way. We score grooves in the first course of asphalt so the second one will bond with it.

Billy Maize
Sep 22, 2008


Awesome. I'm going into Urban Planning (although just starting out, so I'm not entirely positive on what I would necessarily be doing) so all of this is incredibly interesting to me.

What do you think about how auto-centric U.S. cities are? How do you think things will change (more or less auto dependent)?

BigHead
Jul 25, 2003
Huh?

Do you also design red lights and timings of red lights? If so, why the Christ can't every city be designed so that, if you travel the speed limit on major roads, you hit only green lights (like in Anchorage) as opposed to hitting every single goddamn red light (like in St. Louis)?

Are all the pictures roads you've designed, or are they a representative sample?

Do you have a favorite "style" (for lack of a better word) when it comes to designing highways? Do you prefer all straight lines, such as the last picture in the OP, or all curvy circular roads like the other picture?

Do you have a favorite specific city or area you've designed?

Do you deal with construction companies? What happens when a construction company has a terrible terrible reputation - for instance they take three times longer than another other company - but somehow keeps winning the low bids? Do you all grumble about it? In other words, would you analogize construction companies and roads to, say, hamburgers and Morton's vs McDonald's?

What are the best ways to start a career in this line of work?

Do you ever work with other urban planners to design highways that conveniently service certain areas, or is it all about traffic control? Speaking of which, what are several factors you take into consideration when designing a highway?

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


Crackpipe posted:

Have you planned any busways?

Both L.A. and Boston have run into serious problems with the pavement essentially collapsing after a short while. I think the Orange Line in L.A. is on its 2nd re-pavement or so since 2005, while the roadway of the Silver Line tunnel in Boston is literally crumbling back to gravel.

Is this "normal" given the weight of the buses and the fact they're traveling over the same narrow patch of pavement pretty much constantly, or is it just shoddy construction?

We're in the (very long) process of designing the New Britain - Hartford Busway. It's arguably the biggest project in the state, and there are no end to the problems we're having, just in the design phase. It's a well known fact that the traffic composition, which is a fancy way of saying "how many heavy vehicles," is the single biggest influence on fatigue. One semitrailer, for example, can do as much damage as a million passenger cars. Asphalt that's meant to last for 10 years under car traffic (and let's be honest, when money's short, it's more like 20) isn't going to last nearly as long when heavy buses are digging ruts in it.

quote:

Bah, have it your way traffic engineers!

It's not us, we don't care too much one way or another. This is a matter for the pavement management and financial folks.

quote:

Yeah, I was thinking of the sections immediately west of Manchester which aren't in the best shape, but they're older than several typical goons combined.

Like I said, concrete lasts a long time. This section isn't as old as you think, though. That whole area was massively redone in the mid-80s to accommodate all the I-384 and I-291 on-ramps, as well as bring a 40-year-old freeway up to modern standards.

nm
Jan 28, 2008

"I saw Minos the Space Judge holding a golden sceptre and passing sentence upon the Martians. There he presided, and around him the noble Space Prosecutors sought the firm justice of space law."

Cichlidae posted:

Replying to you first since it's an easy question. That's grooved pavement. Asphalt isn't all laid out at once; we put down the base course, then the leveling course, then the wearing course. Often, days go by before the next course gets laid. If you've ever made pottery before, you'll remember that wet clay doesn't stick to dry clay, and you have to score it first to get more friction. This works the same way. We score grooves in the first course of asphalt so the second one will bond with it.
unless there was construction and that's the result of taking off a layer of asphault off concrete berfore you repave it.
In CA, we just abandon that poo poo when we run out of money.

Have you ever done a speed survey? Have they every followed it?

Also, what the gently caress were they thinking with this?
http://maps.google.com/maps?client=...042229&t=h&z=15

If you can't tell, traffic from 62 westbound has to cross 2 lanes of 35w to get to 2 on the other side. (This whole thing is currently being torn up and rebuilt though)

In fact, Minneapolis had a whole bunch of these shared roadway interchanges (for example, move a few miles north on 35W and you get a similar disaster up at 94). They are all a disaster if you get more than 10 cars on the road.
http://maps.google.com/maps?client=...29&z=15&iwloc=A
(The second one I suspect was built that way you save space, but I don't think there was much around 35W and 100 when that was built 50 odd years ago)

nm fucked around with this message at Jul 27, 2009 around 23:23

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


Billy Maize posted:

Awesome. I'm going into Urban Planning (although just starting out, so I'm not entirely positive on what I would necessarily be doing) so all of this is incredibly interesting to me.

What do you think about how auto-centric U.S. cities are? How do you think things will change (more or less auto dependent)?

My country's obsession with the automobile has ruined it for a long time to come. Suburbs are, as I'm sure you're aware, a pretty horrible thing from an urban planning standpoint. Gas isn't going to get any cheaper in the decades to come. Whereas most countries have highly developed urban centers surrounded by farmland, a system that is very well suited to mass transit, the USA is going to have some MASSIVE growing pains. If gas gets scarce enough, and we don't somehow make cars that don't need it, we could very well see abandoned suburbs and a resurgence in urban construction. That's all common sense, though.

What I think is that the average American will figure out, about 10 years too late, that he should live near where he works. Our road infrastructure is going to crumble soon without some major investment, and a lot of that money will get thrown toward rail and bus lines. No matter how much you love your car, if it's three times cheaper to take a bus or train to work, you're going to start thinking.

nm
Jan 28, 2008

"I saw Minos the Space Judge holding a golden sceptre and passing sentence upon the Martians. There he presided, and around him the noble Space Prosecutors sought the firm justice of space law."

Another question, so i stop editing I'll make a new one.
Why does public transit suck so bad in places that should have good public transit. I know about suburbs. But we're talking about cities like Sacramento that had extensive public transit networks in most of the city before they got torn out by GM in the 40s. They were (and are) sufficently dense.
Is it just money? Or is there more to it?

My neighbor is a traffic engineer, i should go yell at him.

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


BigHead posted:

Do you also design red lights and timings of red lights? If so, why the Christ can't every city be designed so that, if you travel the speed limit on major roads, you hit only green lights (like in Anchorage) as opposed to hitting every single goddamn red light (like in St. Louis)?

There's a fun question! I'll make an entire post about signal coordination, rather than hide the answer here.

quote:

Are all the pictures roads you've designed, or are they a representative sample?

Of what I showed in the OP, I've only personally done the Hartford freeway model. I'm only 24; my construction experience is limited to working on one freeway in France, some transportation management, and a few dozen repair/rehabilitation projects in the US.

quote:

Do you have a favorite "style" (for lack of a better word) when it comes to designing highways? Do you prefer all straight lines, such as the last picture in the OP, or all curvy circular roads like the other picture?

I prefer freeways to have gentle curves and slopes. It's much more alluring, more fun to drive, and keeps the driver alert. There's nothing interesting about driving for an hour in a straight line! Realistically, though, the engineer has very little say on where exactly a road goes. That's more dependent on who owns the property it needs.

quote:

Do you have a favorite specific city or area you've designed?

Like I said, I haven't designed much. One city I really like is Chandigarh. It was designed by LeCorbusier, one of the pioneers of urban design. I play a lot of SimCity, and something about its modular nature really speaks to me.


quote:

Do you deal with construction companies? What happens when a construction company has a terrible terrible reputation - for instance they take three times longer than another other company - but somehow keeps winning the low bids? Do you all grumble about it? In other words, would you analogize construction companies and roads to, say, hamburgers and Morton's vs McDonald's?

Unfortunately, we always go with the low bidder. If someone REALLY pisses us off, for example, by faking inspection reports, we blacklist the company. Then the head retires, his son takes the top position, the company changes its name, and it's back to business as usual.

quote:

What are the best ways to start a career in this line of work?

Get a civil engineering degree with a concentration in transportation and pray that someone's hiring in this economy.

quote:

Do you ever work with other urban planners to design highways that conveniently service certain areas, or is it all about traffic control? Speaking of which, what are several factors you take into consideration when designing a highway?

We prefer to build highways proactively, rather than reactively. If a new airport, a big casino, or an industrial park is going in, or if the population's growing so fast that we'll need something in 20 years' time, we will plan the project ahead of time to handle the anticipated need. Unfortunately, around here in New England, there is very little unsettled land and a lot of environmentally protected areas, so we are often forced to work reactively. That means adding lanes where we can, widening roads, etc.

As for design factors, it's a combination of geometrics (we have a lot of standards), available right of way, environmental concerns, and leaving room for future expansion. I'll talk about it more in detail as the thread matures.

Billy Maize
Sep 22, 2008


nm posted:

But we're talking about cities like Sacramento that had extensive public transit networks in most of the city before they got torn out by GM in the 40s.

I just wanted to point out how much this pisses me off. For anyone reading this who hasn't heard about it, go read about the Great American streetcar scandal.

http://en.wikipedia.org/wiki/Great_...reetcar_scandal

It especially pisses me off because I've seen a map with the trolley routes in Salt Lake and they were great. Now I pretty much need to rely on my car.

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


nm posted:

Have you ever done a speed survey? Have they every followed it?

Answering this alone because it's important.

In our theories of design, there is one number that shows up again and again. In France, they call it the V15. Here, we call it the 85th percentile speed. This is the speed slower than which 85% of drivers travel. It's usually 5-10 mph above the average speed. (There are two ways to calculate mean speed, but that's a story for another time.)

This 85th %ile speed is supposed to be equal to another number, the elusive design speed. The design speed is the maximum safe speed for the worst design vehicle on dry pavement. For example, if a freeway has a design speed of 75 mph, then a turnpike double (huge tractor-trailer) could safely negotiate its curves at 75 mph on a good day. Most cars, obviously, could safely go much faster, even in adverse conditions.

And then we have the third important speed, the speed limit. This is ALSO theoretically equal to the 85th %ile speed and the design speed. Sounds simple enough, right?

Every road I have a project on will have a speed survey done. They generally show that the 85th %ile speed is pretty close to the design speed. This is the Northeast, so I wouldn't be surprised if it's a little faster; we don't like to waste time. I try to set the speed limit around that point.

Then Granny McGee and her 40 bingo buddies write an angry letter to the mayor, complaining that people are speeding and how unsafe the road is! Next thing I know, I'm putting up a "25 mph" sign on a road that I know can handle 40 with no problem.

Now lowering the speed limit sounds like it would be pretty safe, right? Welp, buddy, that's why I'm the traffic engineer and not you. Turns out, it's not so much the speed that makes accidents, but the speed differential! When Granny McGee's blustering along at 40 mph, trying to keep up with the rest of the cars, nobody's getting rear-ended. But once she slows down to 25 mph, and the guy behind her is trying to go 50, that's a dangerous situation.

What we see when we check speeds on almost any road, is that the 85th %ile speed is generally 10 mph faster than the speed limit. Does that mean that the MAJORITY is speeding? Probably! Does that mean the speed limit should be raised? Probably! Does that mean you can drive as fast as you think is safe? Well, I already answered that one in the OP. You've gotta follow the laws, no matter how dumb they are.

AAPsel
May 23, 2009


I'm a civil engineer myself, though not a traffic guy. I remember reading reading a comparison of highway capacity per country way back in class though. Most euro countries were able to transport around 1900-2100 cars per hour per lane before congestion set in. Germany somehow managed to outperform everyone at 2500. I think US and Italy were the worst with 1600 or something similar.

Why can a highway handle 50% more Germans than Italians? It's not the roads I guess. Never mind the 50's, the German roads are from the 30's and every exit is outright lethal.

Also, why isn't every junction just a cloverleaf? Why do you need to design a completely new and confusing heap of spaghetti every time two highways meet?

GoonsInDepth
Apr 10, 2006

by Peatpot


Awesome thread, thanks! Very interesting.

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


Now to address the rest of your post.

nm posted:

Also, what the gently caress were they thinking with this?
http://maps.google.com/maps?client=...042229&t=h&z=15

If you can't tell, traffic from 62 westbound has to cross 2 lanes of 35w to get to 2 on the other side. (This whole thing is currently being torn up and rebuilt though)

That's known as Weaving, a very important term in highway design. You're right, it's a substandard design for several reasons. The weaving is due to a left entrance from 62 onto I-35, which is pretty much taboo these days. I-35 is the main line here, so all entrances and exits should be to and from the right side.

quote:

In fact, Minneapolis had a whole bunch of these shared roadway interchanges (for example, move a few miles north on 35W and you get a similar disaster up at 94). They are all a disaster if you get more than 10 cars on the road.
http://maps.google.com/maps?client=...29&z=15&iwloc=A
(The second one I suspect was built that way you save space, but I don't think there was much around 35W and 100 when that was built 50 odd years ago)

In general, when you have multiple routes on the same roadway, it's called a concurrency. Unfortunately, in this case, I-35 and I-94 are both important roads, so there's no obvious priority. Ideally, the ramps would cross over the outer highway so all entrances and exits were on the right, but money and space are important constraints.

Overall, though, that interchange doesn't look too bad. Except for I-94, all the entrances and exits are on the right. The biggest problem there isn't weaving, it's that the ramps are too close together! Anyone out there who lives near a city can laugh at this with me: exits shouldn't be closer than a mile apart. I repeat, you shouldn't see more than one exit per mile, on an ideally designed road. Around here, though, it's not unusual to see 3 or 4 exits in a half mile.

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


nm posted:

Another question, so i stop editing I'll make a new one.
Why does public transit suck so bad in places that should have good public transit. I know about suburbs. But we're talking about cities like Sacramento that had extensive public transit networks in most of the city before they got torn out by GM in the 40s. They were (and are) sufficently dense.
Is it just money? Or is there more to it?

My neighbor is a traffic engineer, i should go yell at him.

Many, many many cities actually had excellent public transportation (referred to as transit in the traffic engineering lexicon) prior to the second half of the 20th century. Light rail, specifically, was quite prevalent around the world. Unfortunately, many jurisdictions (I'm looking at you, California) decided it would be prudent to turn their rail lines into extra lanes for their freeways. After all, gas was cheap, commutes were short, and automobiles were the wave of the future!

Most light rail got replaced by bus in the 30s-70s, but here's the hilarious part: light rail is on the rise again in a big way. Cities all around the world (Europe's at least a couple decades ahead of the USA in this respect) are rebuilding their old tram lines, or making new ones. We figuratively just finished ripping up our old train tracks, and now we get to build them all over again! I love this country.

Billy Maize posted:

I just wanted to point out how much this pisses me off. For anyone reading this who hasn't heard about it, go read about the Great American streetcar scandal.

http://en.wikipedia.org/wiki/Great_...reetcar_scandal

It especially pisses me off because I've seen a map with the trolley routes in Salt Lake and they were great. Now I pretty much need to rely on my car.

Exactly. Boston used to have a much more extensive system, too. I wish engineers in the 50s had had some sense of foresight.

psydude
Mar 31, 2008

Perry'd.


Does anything influence the speed limit other than engineering restrictions?

My dad was an inspector for the Maryland State Highway Administration for 8 years and worked on the construction of a vital interstate spur around Baltimore, so I grew up with an appreciation for the amount of work that goes into designing and building the road networks that people bitch about driving on every day. Kudos for choosing a thankless, but important job.

Edit: Wow, how did I miss that wall of text?

psydude fucked around with this message at Jul 28, 2009 around 00:14

Lord Dudeguy
Sep 17, 2006
[Insert good English here]

Who in the hell thought that the route 395/2 interchange in Connecticut was a good loving idea?

:edit: Seriously only about 50 yards to merge-in/merge-out

nm
Jan 28, 2008

"I saw Minos the Space Judge holding a golden sceptre and passing sentence upon the Martians. There he presided, and around him the noble Space Prosecutors sought the firm justice of space law."

Cichlidae posted:

Answering this alone because it's important.

In our theories of design, there is one number that shows up again and again. In France, they call it the V15. Here, we call it the 85th percentile speed. This is the speed slower than which 85% of drivers travel. It's usually 5-10 mph above the average speed. (There are two ways to calculate mean speed, but that's a story for another time.)

This 85th %ile speed is supposed to be equal to another number, the elusive design speed. The design speed is the maximum safe speed for the worst design vehicle on dry pavement. For example, if a freeway has a design speed of 75 mph, then a turnpike double (huge tractor-trailer) could safely negotiate its curves at 75 mph on a good day. Most cars, obviously, could safely go much faster, even in adverse conditions.

And then we have the third important speed, the speed limit. This is ALSO theoretically equal to the 85th %ile speed and the design speed. Sounds simple enough, right?

Every road I have a project on will have a speed survey done. They generally show that the 85th %ile speed is pretty close to the design speed. This is the Northeast, so I wouldn't be surprised if it's a little faster; we don't like to waste time. I try to set the speed limit around that point.

Then Granny McGee and her 40 bingo buddies write an angry letter to the mayor, complaining that people are speeding and how unsafe the road is! Next thing I know, I'm putting up a "25 mph" sign on a road that I know can handle 40 with no problem.

Now lowering the speed limit sounds like it would be pretty safe, right? Welp, buddy, that's why I'm the traffic engineer and not you. Turns out, it's not so much the speed that makes accidents, but the speed differential! When Granny McGee's blustering along at 40 mph, trying to keep up with the rest of the cars, nobody's getting rear-ended. But once she slows down to 25 mph, and the guy behind her is trying to go 50, that's a dangerous situation.

What we see when we check speeds on almost any road, is that the 85th %ile speed is generally 10 mph faster than the speed limit. Does that mean that the MAJORITY is speeding? Probably! Does that mean the speed limit should be raised? Probably! Does that mean you can drive as fast as you think is safe? Well, I already answered that one in the OP. You've gotta follow the laws, no matter how dumb they are.
Thanks
I worked in the only city in the world that ever seemed to do this right.
This was a fairly large suburban city in the twin cities. We had fairly wide two lane streets through nominally residential neighborhoods (aprtaments far set back from the street). 30mph So the city got enough bitching about the 30 mph streets. They wnated 25mph.
They hired an engineer to do surveys on the streets. They came back as 35 mph. The city then made the 35mph. No bitching about safety, they just took it an ran with it.
Strangely enough, people still bitch and the same thing happens.
I had no issues conmvicting people of speeding there.

Silver Falcon
Dec 5, 2005

Citizen of Zada

Steppo posted:

Who in the hell thought that the route 395/2 interchange in Connecticut was a good loving idea?

:edit: Seriously only about 50 yards to merge-in/merge-out

Oh poo poo I know that road! Yeah I hate that exit. HATE with a passion. One of these days I'm going to call up the DOT and complain. (I've heard from a reliable source that they're required to at least listen to you and investigate your complaint).

Oh yeah and the worst part about that road is it's the shortest route back home to visit the in-laws.

Cichlidae
Aug 12, 2005

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AAPsel posted:

I'm a civil engineer myself, though not a traffic guy. I remember reading reading a comparison of highway capacity per country way back in class though. Most euro countries were able to transport around 1900-2100 cars per hour per lane before congestion set in. Germany somehow managed to outperform everyone at 2500. I think US and Italy were the worst with 1600 or something similar.

Why can a highway handle 50% more Germans than Italians? It's not the roads I guess. Never mind the 50's, the German roads are from the 30's and every exit is outright lethal.

Also, why isn't every junction just a cloverleaf? Why do you need to design a completely new and confusing heap of spaghetti every time two highways meet?

Let me answer your first question really quickly, because the second one is awesome and deserves its own post. The capacity of a freeway depends on the traffic composition I talked about earlier, the grade (slope) of the road, and the geometry of the roadway. On our freeways in Connecticut, we can get 2500 pcplpm (passenger cars per lane per mile) in good conditions, and 1500-1800 when the road is under construction.

One of the many paradoxes of traffic engineering is that traffic flows best when the road is mildly congested. Time to get technical!

We measure how smoothly traffic is flowing by LOS (Level Of Service). A is the best: you're the only one on the road, you can go as fast as you want. At LOS B, there are a couple other cars, but they don't impede you much. LOS C is a bit rougher, and you have to be careful when changing lanes. LOS D is quite stressful. Everyone's moving at a good clip, but you don't have much liberty. LOS E, you're going 10 below the speed limit and everyone's crammed in like sardines. Finally, at LOS F, it's stop-and-go, or just plain 'stop'.



You can see here that speed goes down as volume (and density) go up. However, since there are many more cars on the road at LOS E than at LOS A, even though they're going more slowly, the road is handling more traffic and therefore more efficient. Fascinating, eh? That's why a German road that's often congested can actually be better than an Italian road that nobody uses. Or maybe the Italians just can't drive.

Cichlidae
Aug 12, 2005

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psydude posted:

Who determines the speed limits on highways?

My dad was an inspector for the Maryland State Highway Administration for 8 years and worked on the construction of a vital interstate spur around Baltimore, so I grew up with an appreciation for the amount of work that goes into designing and building the road networks that people bitch about driving on every day. Kudos for choosing a thankless, but important job.

Edit: Wow, how did I miss that wall of text?

Much as I'd like to say the engineer sets the speed limit, Granny McGee and her bingo buddies have about 100X more influence than me.

Cichlidae
Aug 12, 2005

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Steppo posted:

Who in the hell thought that the route 395/2 interchange in Connecticut was a good loving idea?

:edit: Seriously only about 50 yards to merge-in/merge-out

Yes, I almost got into a crash at that very spot. So now's a great time to talk about WHY CLOVERLEAF INTERCHANGES SUCK!

Here is a handy diagram to demonstrate why cloverleafs introduce weaving.


Any time you have an on-ramp followed closely by an off-ramp, weaving will be an issue. Mister Red in the diagram needs to get to the left, so he doesn't get trapped into taking an unintended exit. Mister Blue, however, wants that exit. See how they're pretty much on a collision course? That's weaving, and it's a major problem in highway design. Mister Green here isn't helping matters; he wants to take the next exit, and he's going to shift to the right, not paying much attention to Mister Red moving into that same lane.

Cloverleafs are bad because they introduce FOUR of these weaving sections into a single interchange. That's a massive safety issue. We'd much rather replace them with diamond interchanges or partial cloverleafs (parclos), even if it means lowering capacity.

Oh, one last thing: cloverleafs have no expansion potential, since putting two lanes on a loop ramp (the 270 degree ramps that make the "leaves") is next to impossible, increasing the ramp design speed by a few mph means making the interchange MUCH bigger, and cars have to go way out of their way instead of going in the direction they want.

Cichlidae
Aug 12, 2005

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nm posted:

Thanks
I worked in the only city in the world that ever seemed to do this right.
This was a fairly large suburban city in the twin cities. We had fairly wide two lane streets through nominally residential neighborhoods (aprtaments far set back from the street). 30mph So the city got enough bitching about the 30 mph streets. They wnated 25mph.
They hired an engineer to do surveys on the streets. They came back as 35 mph. The city then made the 35mph. No bitching about safety, they just took it an ran with it.
Strangely enough, people still bitch and the same thing happens.
I had no issues conmvicting people of speeding there.

I'd love to have the cities and towns back me up like that. There's so much animosity between the residents and the people who set the speed limits! They feel like the speed limit is the reason that people speed. Yes, lowering the speed limit in conjunction with strict enforcement will make 80% of drivers go slower, but the 20% who don't are the ones who drive the fastest anyway.

potato of destiny
Aug 20, 2005

Wheeeeee!



So bookmarking this thread.

The question I had goes to your discussion of the 85th percentile speed. I've heard of ideas being kicked around of setting a variable speed limit based on the 85th percentile of whatever the current average traffic speed is; has there been any serious thoughts of implementing something like this in the US? (I think I remember reading somewhere about a system like this in Germany).

Tom Steele
May 17, 2009


This is actually very interesting and something i've always wondered about. Can you do that post about traffic signals please?

Thanks OP

Cichlidae
Aug 12, 2005

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MAKE RED LIGHT


potato of destiny posted:

So bookmarking this thread.

The question I had goes to your discussion of the 85th percentile speed. I've heard of ideas being kicked around of setting a variable speed limit based on the 85th percentile of whatever the current average traffic speed is; has there been any serious thoughts of implementing something like this in the US? (I think I remember reading somewhere about a system like this in Germany).

Yes, we have some systems that can change the speed limit. I remember seeing some while driving through New Jersey (not a journey I'd recommend to anyone else). I think they respond to the road conditions more than the speed of the drivers. I know on the Autobahnnetz, variable speed limits are common because the roads are much "smarter." There are cameras and sensors all over the freeways, keeping track of road conditions, speed, occupancies... we just don't have the money here to install that sort of monitoring everywhere.

There are some places where we're experimenting, but most of the work I've done in transportation management has focused on reacting to incidents, not to normal operations.

Cichlidae
Aug 12, 2005

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Tom Steele posted:

This is actually very interesting and something i've always wondered about. Can you do that post about traffic signals please?

Thanks OP

Since I promised it earlier, here's how coordination works!

COORDINATION 101



This is called a time-space diagram. The little figure on the left represents what we call a corridor. This is a major road, and we want to keep the traffic moving on it. At each of the four intersections shown there, there's a signal. Since this is the main road, it'll get about 50% of the signal's cycle as green time, and, logically, the other 50% is red. These red times are shown to the right as horizontal bars. It should be pretty obvious what their widths represent if you look at the axis labels.

Now, we've got vehicles driving through that corridor, shown as blue and magenta lines. If X represents time and Y represents space, then the slope of the lines represents the speed of each vehicle. Each time the car has to stop (and we don't bother curving the lines to represent acceleration and deceleration), the line goes horizontal. Makes sense, right?

So, we want as few stops as possible. In this diagram, the lights aren't coordinated very well, since there are a lot of stops. These stops waste time and gas! Therefore, we move around the red times and offets between lights and get this:



You see? Much fewer stops. All we had to do is move things around a bit. The number of cars that can get through the corridor without stopping is called the bandwidth. Unfortunately, this sort of situation is VERY hard to achieve in real life, but that's a topic for Coordination 102.

Cichlidae
Aug 12, 2005

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Let me tell you a little more about my first transportation engineering job. I worked in what's called a TMC (Transportation Management Center.) How does one manage transportation, exactly? Well, check this out:



It's basically a bunch of "operators," sitting around in front of some BIG TV screens and watching video feeds from highways around the state. We had nearly 100 cameras when I was there, along with several dozen electronic signs, a few radio towers, and live feeds to TV and the website.

I had a lot of fun at that job. We'd sit around for 8 hours, watching cars crash and then responding to the crashes by putting up advisories on the website and radio and making messages on the signs to warn people of the incident ahead. I saw some amazing incidents in my time there, stuff you only see on the news, and some horrible things, too. I could fill a whole thread with the things I saw there... and maybe I will!

I learned a lot there, and here's something everyone should know about incident management. First off, if you see an electronic sign on the road, it's either the state's or a contractor's. Never trust a contractor's sign. The state's sign, though, should be the easiest and most reliable way to find out about an accident ahead. If it says to seek an alternate route because of an incident, you should really consider it.

The phone travel advisory (dial 511 in most states, try it now!) is also usually up-to-date. It carries scheduled construction information, as well as accident information. Most DOTs also have a website that shows where there is an incident. These are all pretty reliable.

But the radio, that's a different story altogether! They're a TMC's lowest priority, because they generally have horrible reception, take a long time to scroll the 60 seconds of station callsign BS and routine advisories, and usually have a delay of 5-10 minutes or more. We have to record those voices ourselves, and that takes precious time that we need for alerting the police, setting up messages on the signs, and blacking out the cameras so that our good friend Granny McGee doesn't turn on the traffic channel and see her grandchildren smeared across the high-speed lane on I-95.

Speaking of the Highway Advisory Radio, it's entirely possible you've heard my anonymous, velvety voice before and not even known it! Imagine that.

teejayh
Feb 12, 2003
A real bastard

Have you heard of the Diverging Diamond Interchange, and if you have, what do you think of it?

Panzerfaust
Jul 8, 2006

Just when you think that there's nobody around to kill you, BOOM, remote controlled Panzerfaust outta nowhere.

Thanks for this thread, Cichlidae. I'm moving to Toronto at the end of next month to start an urban planning degree, so all of this is very interesting, though my interests lie more in the transit sphere than the highway sphere.

Regarding your comments on the death of streetcar systems in North America, Toronto was lucky enough to have a citizen's coalition that stopped the abandonment policy in 1972. Since then several new lines have been constructed and upgraded, and there's a plan in the works to build several new streetcar lines and a large number of LRT lines all around the city. While I have my own objections to large portions of the plan (it's "LRT everywhere" instead of "LRT where it makes sense"), I'm glad to see that the city government's attitude towards streetcars has totally about-faced since 40 years ago without them ever having gotten rid of the legacy system to begin with.

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


teejayh posted:

Have you heard of the Diverging Diamond Interchange, and if you have, what do you think of it?

I love them. Crazy stuff like the DDI makes my job interesting! Let me draw out a quick diagram for those who don't know what I'm talking about, though:



This is a standard diamond interchange. Note that there are two signals, each of which has at least three phases because of the two left turns at each one. It's a pretty standard kind of interchange, though not the safest, strictly speaking.



Here is a DDI! It has the same number of signals, but each one only has 2 phases, making them more efficient. Additionally, there are no left turns. The weird part is that, on the overpass, traffic's driving on the "wrong" side of the road. Pretty cool, eh?

I'd love to see more of these, but, unfortunately, it'll be a long time before they show up here. Americans are just too darn stupid to use complicated interchanges like that! Just about anybody can get a driver's license here, which means we always have to design for the stupidest motorist. You'll see that theme pop up again and again.

Edit: There are a couple of these in France. Also, we have a half-formed DDI right in Rhode Island! It doesn't have the best reputation, unfortunately.

Cichlidae fucked around with this message at Jul 28, 2009 around 01:52

Cichlidae
Aug 12, 2005

ME LOVE
MAKE RED LIGHT


Panzerfaust posted:

Thanks for this thread, Cichlidae. I'm moving to Toronto at the end of next month to start an urban planning degree, so all of this is very interesting, though my interests lie more in the transit sphere than the highway sphere.

Regarding your comments on the death of streetcar systems in North America, Toronto was lucky enough to have a citizen's coalition that stopped the abandonment policy in 1972. Since then several new lines have been constructed and upgraded, and there's a plan in the works to build several new streetcar lines and a large number of LRT lines all around the city. While I have my own objections to large portions of the plan (it's "LRT everywhere" instead of "LRT where it makes sense"), I'm glad to see that the city government's attitude towards streetcars has totally about-faced since 40 years ago without them ever having gotten rid of the legacy system to begin with.

Yes, there aren't many cities that can boast an original system. Eastern Europe's done extremely well with their light rail; on a system 40 years old, a Hungarian (if I remember correctly) tram is carrying 400,000 passengers per day on a single track! Compare that to a standard freeway lane in great shape (25,000 cars per day), that single track is carrying the equivalent of 16 lanes.

"LRT everywhere" might not seem like that sound of a strategy, but with efficiency like that, it's hard to argue with putting it wherever you can reasonably afford. A city half the size of Toronto, Brussels, has multiple metro and premetro lines, as well as several trams and a dense network of buses. My approach to mass transit is that more is better. It's very hard to saturate rail lines, and more light rail means more direct trips and shorter headways.

Edit: Check this out. 24 tram lines, 6 metro lines.

Cichlidae fucked around with this message at Jul 28, 2009 around 02:04

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FyreStar
Jun 18, 2004
SILENCE, BLATHERING TOADIES!

Great thread, OP.

I realize you're relatively new to the field, but how has the rise of technology affected design and traffic analysis? It seems like computer modeling would be an incredible time saver.

And, along those lines, where do you see it going in the next 20 years (or longer)? Do you think technological advances (either in paving/planning or car design) will help with the growing pains?

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