I am a licensed professional civil engineer who specializes in traffic engineering, illumination design, and ITS. I've also done a good bit of land development engineering. So ask me some questions if you want to. I know Chris Pratt is hogging all the spotlight right now, but if you can tear yourselves away from him for a minute to focus on rampart traffic engineering maybe you'll learn something about the fascinating world of traffic engineering, illumination, ITS, and some land development engineering.

Edit: here's some proof: my Texas engineer pocket card.

Another edit: I think I've answered all the questions that are currently up, and I'm pretty tired so I think I'll turn in. I'll check back tomorrow morning and answer anything else that comes up.

Comments: 86 • Responses: 36  • Date: 

FatherSpacetime18 karma

I have been hearing by word of mouth that roundabouts are more efficient and can be safer than traffic lights. Is this true? If it is, why are we not implementing more roundabouts into our traffic infrastructure?

gm218 karma

You heard right - in general, roundabouts are safer and more efficient than signals. If the driving population in your area is savvy to the roundabouts and how to navigate them, then they can be useful.

The reason you don't see more of them in America is that the typical driver isn't used to seeing them and they complain about the mere thought of implementing them. But studies show that they will reduce crashes, and there is the obvious benefit that they don't take any electricity to run and they don't go into flash in the morning rush hour to cause delays because of a power spike.

I think you'll see more and more of these being implemented, depending on where you live. In Texas, they are slowly becoming more popular.

Edit to say: it is important to light the roundabouts correctly, and design them to allow proper turning radius, and they do consume more right-of-way space than a signal does, in general, so those are also some reasons why they have been a little slow to show up. But I think going forward, there will be more of them because the benefits outweigh the costs.

Jevo_4 karma

I have a follow up comment to this. I am civil engineering student, and has had some involvement with traffic engineering so far. I was told by an advisor on a project I did, that roundabouts are better, up to a certain traffic intensity. Basically once the roundabout gets gridlocked, it loses many of its advantages in terms of allowing traffic flow, and at that point it is more efficient to use a regular light intersection. But in many situations the roundabout will be better, and I think it is very true, that there is a lot of potential for them in the US. I am from Denmark, and we have been using roundabouts extensively for many years now, and for a long while the general wisdom was that roundabouts are always better, if there is space to build one. But over the last few years, most have started to see that the world is a bit more nuanced than that.

I hope what I wrote makes sense, I don't really know the English terminology.

gm22 karma

Yeah, that's a good point, they will break down if they are saturated, but typically you'd do some studies to figure out what kind of traffic to expect within the design life of the facility and design it to accommodate that traffic.

CollatrlDamage11 karma

If I push a button at the crosswalk, how influential is it on the light itself?

Also, are weight sensors or some type of radar more effective at stoplights?

gm220 karma

The pedestrian buttons are designed to work thusly:

You press the button. That sends a signal to the controller that there is a ped who wants to cross the street. In most cases, it will wait until the time in the cycle when the corresponding vehicular movement is scheduled to get some green time. At that time, you will see the walking man sign on the pedestrian signal head to tell you when to walk.

What it won't do is stop opposing traffic early so that you can cross. It just lets the controller know that there is a pedestrian crossing the road, and that it should not allow the cross-street phase to terminate early if there are no cars there to use it. It will time your pedestrian interval so that you can cross the street.

But, if you are crossing the side street instead of the main street, it is likely that the main street was already going to get its full amount of green time anyway, so your pressing the button doesn't do much except show the walking man indication on the pedestrian signal head to tell you when it's safe to walk. The timing of the green intervals won't change.

I guess the answer to your first question is that it may change the operation of the signal a bit, if you are crossing the main street instead of the side street, but otherwise it doesn't change the operation of the signal much. Assuming that you are at a fairly heavily traveled intersection, at least. If you are at a signal where there is very light traffic, then you might get extra green time to cross as a pedestrian that normally wouldn't be given, because the streets at that intersection may not be programmed to allow so much green time for so few vehicles.

As for the weight sensors - they are used in some places, but they depend on the pedestrians standing at a known location, and they are prone to failure because people like to hit the button and then kind of mill around, and they may not stand on the correct spot to trigger the pedestrian phase.

FockSmulder1 karma

I thought he was talking about weight sensors for cars. I didn't know there were any for pedestrians.

In my town, there are quite a few intersections with advance left turns for people coming from one direction. But they give the advance green even when nobody's there: 4 way intersection; you arrive in the lane to go straight, triggering the lights to change; nobody is sitting across the intersection from you, but you look up to the lights above you and see that these phantom cars are getting an advance green.

Is there a simple solution to this? Can I walk in to some office some place and say "Here's how you can save commuters in this town several hours a month." and have them enter a new line of code or something and fix it? I'm guessing not, but is there a reason why these inefficiencies don't ever change?

gm22 karma

Yeah, call the traffic office and say that you believe that a detector is out at whichever intersection. It's typically a hardware failure and sometimes they don't know about it, or maybe they don't have budget to fix everything so the ones getting complained about take priority.

Amator10 karma

How fictional are movies like The Italian Job that show someone hacking traffic patterns? Do you get a lot of failed hacking attempts on your servers?

gm24 karma

If you mean how practical would it be to have a series of signals turn green for a few seconds while a car goes by, then turn back to red immediately... not very, at least not as portrayed in the movie. It might be possible, if you were able to both compromise the network and you understood how to program traffic signals. The signal controller software can be pretty esoteric, depending on what kind of controller.

You'd have better luck getting a preemption emitter like fire trucks have... that could get pretty close to the effect from the movie, but getting the signals to change back to red would take some thought. Maybe you could have another emitter on the back of your car and shoot the signal from the other direction after you'd passed.

The signal will always time its minimum green after a preemption, so depending on what the time setting is, you might could do it.

capn_yarrgh8 karma

Why can't you traffic engineers ever get stoplights timed correctly?

gm217 karma

Those stoplights you refer to were designed by engineers in a competing company.

Seriously, there are a bunch of reasons, and most of them have to do with not making life miserable for traffic on the side streets. We could get perfect progression but it would require a 6 minute cycle length, and that would suck if you just wanted to pull out of your neighborhood and go to the donut shop, right?

k_stokes1 karma

Hmm.. I can't help but think:

-6 minutes would assume you're the first car to trip the sensor. If there were a somewhat steady trickle of cars coming from that street, you would statistically wait an average of 3 minutes for the light to change as there would likely be other cars which have already arrived at the intersection. And if that makes a major street flow smoothly, pros > cons ?

gm21 karma

Well, keeping in mind the main street is going to always get the lion's share of the green time, I'd say it's more likely ~4-5 minutes of average wait time. Either way, it's a pretty long time. Your typical urban cycle length is on the order of 90-150 seconds, so we're talking about tripling that. Queues lengthen, pedestrian wait times lengthen, etc... I think you'd get a lot of calls.

zudnic6 karma

When stuck in traffic, what behaviors can I adopt to do my part to help dislodge the jam? Not switching lanes, trying to maintain a constant speed, etc? Also in an interstate traffic jam, is there a lane that tends to move fastest?

gm28 karma

I'd say that it depends on what is causing the jam. The #1 thing that would help prevent delay is if people would stop rubbernecking things on the other side of the freeway. Other than that, probably not a lot - if one of the lanes on your side is shut down, there's going to be delay no matter what you do. Changing lanes and stuff has a minimal impact on the overall flow.

If the interstate is jammed, I don't know that one lane vs. the other moves any differently, with the obvious caveat that if the outside lane is stopped, the inside lane might move a little faster, and vice-versa.

During free-flow traffic, the middle lanes tend to move a little better. Usually the right lane is kind of congested because of weaving (exiting traffic merging with entering traffic) and the left lane tends to get crowded because people think it's the fast lane. So that leaves the middle lane(s) to flow a bit better.

kevinkit1 karma

Speaking of rubbernecking, there was a severe case going down I-30 earlier this week. There was an accident with a partially charred car off in the right shoulder, and traffic jammed up completely for about 1.5 miles. Once we got to the accident, there were a lot of people with their emergency lights on, just parked in the middle of the interstate, just rubbernecking. Once we passed the accident (being in the left lane is the only reason we got through), the interstate was completely empty as far as we could see, with the exception of 3 cars. 3 cars. Is there any way you can design roads to discourage rubbernecking?

gm21 karma

Well, see my response to /u/Kevinkit above. Other than that, you're talking about making concrete traffic barriers high enough that no one can see over them, and that's a lot of concrete expense... personally, I think you could justify it, but on the other hand, I suspect once business owners got wind of it, they'd complain because now drivers can't see the Cracker Barrel (or whatever) on the other side of the road.

And unfortunately, that's a real consideration engineers have to make.

dooony6 karma

Hi! I'm a mechanical engineer from Sydney, Australia and find traffic engineering fascinating. Especially how braking downstream can cause traffic jams upstream even when there's no "blockage". Example Can you explain this in more detail and how it is managed?

I have visited Tokyo on a few occasions and always marvel at how well traffic is managed there. At big intersections there are rarely many cars waiting at red lights, but when the light turns green a stream of cars seems to turn up at just the right moment. I imagine they have some very clever algorithm or interconnected system managing traffic flow, but i'm only guessing. Do you know of any examples of any countries or cities who manage traffic really well or interesting systems used around the world?

gm211 karma

Hey brother. And might I add, damn you, because I had just finished my drink and no one had replied in a while, so I was going to bed (it's nearly 2 AM here.) But now that another question has surfaced... I must respond.

So, let me take your question by points:

Hi! I'm a mechanical engineer from Sydney, Australia and find traffic engineering fascinating. Especially how braking downstream can cause traffic jams upstream even when there's no "blockage".

At the 0:12 point in the video, watch the white car slow for no apparent reason. I think he may have been instructed to do so, but no matter - he slowed. The guy behind him slows, and the guy behind him slows some more, and so forth - it causes a "shock wave". That is a real term traffic engineers use, and we can calculate the speed of propagation of the shock wave. There is also a shock wave for acceleration when things speed up again. I think of it like a Slinky - it slows down in a wave, and it speeds up in a wave, too.

The way we manage it is to try to not let it happen in the first place... people don't want to crash in traffic, so there's really no way to stop the shock wave from happening if a big sudden slowdown occurs. If the incident is large enough, it may help to have dynamic message signs (DMS) on the road to inform drivers ahead of time that there is an incident ahead. But that's mostly to prevent secondary incidents, not to solve the problem itself.

If your question is why does the white car slow down in the first place... that's a hard question to answer. It could be a lot of things - maybe a dog ran out in the road, maybe (s)he was looking at the radio, etc.

Edit I wanted to add some specific detail to this: the most common cause of slowdown and the shockwave effect is too many interchanges/ramp entrances on the freeway. I can't really comment on roundabouts because that isn't my area of expertise, but I know a lot about freeway traffic and access management is problematic.

I have visited Tokyo on a few occasions...

I've never been to Tokyo, but Anthony Bourdain makes it sound nice. I have a couple of thoughts about how to explain your experience there:

  1. Tokyo is one of, if not the most, congested cities in the world. Every tiny little bit of improvement to traffic there corresponds to a large increase in mobility and commerce. So it is very much in the interest of the city's leadership to make sure things work as efficiently as they can. They no doubt invest a lot into traffic engineering projects to make sure things are optimized.

Now, having said that, I don't know where you were driving in Tokyo. Were the streets one-way? Were they 6-lanes with lots of right-of-way? We traffic guys can do a lot if we have some of constrictions removed, and those are two of the biggest constrictions.

Do you know of any examples of any countries or cities who manage traffic really well or interesting systems used around the world?

Places that can invest in public transit do well. Places I've been that have good traffic in the subway, but do not have good traffic conditions on the roadway network:

  • London
  • Paris
  • Rome
  • Frankfurt (I think, didn't ride subway in Frankfurt, and can't swear that there even is one)
  • Munich
  • New York
  • San Francisco (they have BART, no subway here)
  • Dallas (we have the DART and it's ok)

Basically, every world class city.

Oddly, I don't remember Dublin having terrible traffic, but that may because I was drinking a lot of Guinness and riding the bus everywhere without a care.

Edit: I realize a made a list consisting of one item. That's... what I meant to do.

Second edit: I put on Men At Work while writing this response, so thanks for that. Cargo is one of the best albums ever. If I ever ask you a question, I'll expect that you put on ZZ Top or maybe Buddy Holly.

edbooya3 karma

NYCDOT employee clocking in (I am an engineer, but not in traffic management), hey! Nothing technical to add, just an agreement about the state of our traffic management. Despite the nonstop traffic jams, if you ask anyone in that division (or in City government in general) though, you'd be told that we're the best.

gm23 karma

Keep fighting the good fight, brother!

VirtualWork1 karma

Dublin uses SCATS which seemed like an awesome little traffic management system when I was studying transportation there. Dublin can manage to get fairly congested though during rush hour, little medieval roads that were never really designed for such volumes of traffic. That being said, there is a high usage of public transport which succeeds in taking a lot of cars off the road. I read a statistic in a Dublin newspaper once that 85% of people who go into the city center use public transport.

gm21 karma

Interesting, SCATS is a computerized traffic management package, I take it?

We rode the bus a lot in Dublin - Temple Bar, the Guinness brewery, the old Jameson distillery, etc. :)

mothman6075 karma

What's the deal with cloverleaf highway intersections? They always freak me out since I'm given very little space to merge and speeding up on a tight curve can be tricky (especially in bad weather).

gm210 karma

The deal with cloverleaf intersections is that they are cheaper to build than a full interchange with direct connect grade separation bridges. That's why DOTs like them. They provide a reasonable level of service for less money. Your merging anxiety isn't their concern, baby.

Brewe5 karma

Are you really good at Cities: Skylines?

gm28 karma

No! I'm not very good at C:S. I've only done like two cities, but I wish they'd implement a way to exert more control over the traffic. Every intersection has a signal and there's no way to control the timing. That would be a good upgrade once you are a certain size, to be able to add a traffic engineering service, but it might be hard to program.

Zhabba_Zheeba5 karma

Ho ho, do I have just the thing for you!


OK, I pretty much didn't make it past Algebra 2 in high school so I have no hope as a real-life traffic engineer, but when I got this mod I spent probably close to four hours just tweaking intersections in a couple of my cities because it was so much fun. I'd suggest you try it out and it does appear to be an actively maintained mod.

gm23 karma

Noted, I'll check that out.

FockSmulder4 karma

Hey, how often does Braess's paradox come up?

For those who don't know what I'm talking about, it's (very basically) the idea that you can increase traffic efficiency in some circumstances by removing a road.

What sort of road networks is it more likely to happen with? Thanks.

gm22 karma

After thinking about this question for a few minutes, I think it can be compared to an access management policy. You don't want to let every single business construct a driveway onto your street, and you don't want unrestricted left turns into and out of driveways, because doing so increases accidents and congestion.

So you do things like build medians and make people turn right into and out of driveways, improving safety decreasing congestion by making people drive a little bit further.

So if you want to extend the analogy to streets instead of driveways, you can definitely argue that an arterial with 10 residential street intersections will experience a better level of service than the same length of arterial with 20 intersections. Remove some of them, force people to use more of the capacity on the ones that remain, and the system as a whole performs better.

Maybe that didn't directly relate to Braess's theorem, but it's a pretty good analogy, I think, and one that we do encounter a lot.

DrFrankenstein902 karma

I don't know if you have an opinion or not on this, but I'll give it a try… As Texas has implemented it pretty widely, what is your stance on ClearviewHwy as compared to FHWA alphabets for signage?

Many people I know dislike it and when say that it's harder to read, but personally, I like it better and think they just don't like change and/or haven't gotten used to it.

gm21 karma

I like Clearview, I've never thought it was hard to read at all. I agree that people just like to complain about things.

TheAuth0r2 karma

How much money do you make, or at least the ballpark?

gm22 karma

Low 6-figures, with 12 years experience.

agentdarko1 karma

Hey there! Ever heard of Kelly Square in Worcester, MA? You might find it amusing:


gm21 karma

Ha, no I haven't seen that one. Kind of hard to imagine how they arrived at that - did they dust their hands off and say, "that should do it!"

Honestly, I'm sure it got built piecemeal over the years and they put in whatever would fit. Still pretty funny.

kevinkit1 karma

Hi there!

Is there any road design features that have been thought of, tested, or put into practice in order to discourage rubberneckers? They are the bane of going on roads!

gm21 karma

Yeah, actually there is one thing I've seen that works. It wasn't technically designed to prevent rubbernecking, but instead to prevent the sun from shining into drivers' faces, but it really works the same way. You know sometimes there are concrete barriers between the lanes, right? Well, in some places where the angle of the road is just right, the sun will shine directly into drivers' eyes at certain times of the day.

So the design I saw was to install these paddle type plastic things on top of the concrete barriers that are angled in such a way to block the sun when it is low enough to be a problem. This has the extra effect of preventing a view of the other side of the highway except for at 90 degree angles (or close to it.)

That prevents rubberneckers. But it's probably kind of expensive to do that mile after mile, and so you mainly only see it when there is a need to block the sun glare.

kevinkit1 karma

I've seen these paddles before, but now I know why they are there. Thank you! I would like to have a discussion, but a powerful thunderstorm is about to roll in and I might lose power, so thanks for your time!

gm21 karma

Sounds like we're in the same area (Dallas?) - I hear thunder rolling in too.

I need to get to bed soon anyway, but send me a PM if you want and I'll answer it when I can. Maybe tomorrow night, or if I have some spare time at the office tomorrow maybe I'll check in there.

DetroitHustlesHarder1 karma

Not sure if you're a videogamer, but whats your impression of video games like Sim City/Cities:Skylines (the latter of which is pretty dang amazing, if you ask me) in terms of city/traffic planning accuracy & enjoyability in terms of aesthetics?

gm21 karma

Someone asked about this below... I played c:s and thought it was pretty good, but I commented that I wish there was a way to exert more control over the traffic system. He responded with a link to a steam workshop mod that lets you do just that.

I haven't had any time to check it out, but it looks nice! Overall from a city planning perspective, the sim seems more thorough than any of the others I've played over the years, and it can certainly consume a lot of your time :-)

I wish I had a more complete perspective on it, but with two little kids at my house I don't have much time to play it.

Swaayyy1 karma

How did you manage to get pass all those math classes in college? I'm barely surviving in college now and I'm a computer science major

gm26 karma

I've always been pretty good at math, but differential equations (the calculus class you take after Cal III) was pretty tough. Two rules: always go to class, no exceptions, and always attempt the homework, even if you don't know where to start. Figure out where you've gotten stuck and ask the professor to help you.

synergistali-1 karma

Probably unnecessary but diff eq is not calculus at all

gm25 karma

To me, if you integrate or differentiate, it is calculus.

blinlim1 karma

I live in Boulder colorado. In one of the main streets there is a pedestrian crosswalk that cuts through one of the main roads. In the past, there used to be a button that, when pressed, would initiate flashing red lights to oncoming traffic to indicate people passing. Now, however, the button has been replaced by a plastic arrow (???) which is difficult to press. To you have any idea if this has just started working automatically or?

gm23 karma

You can test it - go there and don't press the button, and see if the crosswalk activates after a minute or two. If not, it still needs to be actuated.

Chances are that the button was replaced to upgrade the signal to a Accessible Pedestrian Signal (APS). Those are mandated now, and they typically have audible messages for visually impaired pedestrians. If the button is physically difficult to press, it may be stuck or dirty. Call your local traffic office and ask if they can check it, because it shouldn't be hard to actuate the signal if it is working properly. If people take one thing away from this AMA, I hope it is that they should call the local traffic engineering office and ask questions. They are there to help, and they don't mind citizen calls at all (as long as you are reasonably polite.)

The way you describe it makes it sound like a mid-block crossing (ie, not at a regular traffic signal.) Is it a Hawk crossing like this one?

Maccabeus20101 karma

Apologies if this is too late, but how do you feel about the usage of Level of Service as a measure of road quality? Additionally how do you feel about our recent road funding mechanism controversy with the Highway Fund? Thanks!

gm21 karma

Not too late, but I'm getting ready to go visit my grandmother and your question is more in-depth than I have time for at the moment. I'll think about it and respond tonight.

gm21 karma

OK, so there are different ways to define level of service. For a traffic signal, you normally use the control delay (ie, delay in seconds per vehicle caused by the signal itself.) For a freeway, a lot of times you use operating speed. If you wanted some other metric, you could define "level of service" as anything you wanted.

If you mean do I think we should have something more specific than "level of service C" then my response is probably no - it's just a way to describe how the facility is working, in a general way, which is usually sufficient for things like reports to other engineers.

You can always provide more detail if it is needed.

I'm not sure which controversy you mean about the highway funding - there are several. The gas tax needs to be higher and maybe indexed to inflation, but that's considered regressive. Toll roads aren't very popular, but they are probably not going anywhere because states just don't have the money they need to fund big projects. Maybe the answer can be a combination of things - a one time adjustment to the gas tax, roadway impact fees to developers, or perhaps tie the tax to property evaluations, with a certain amount allowed to be deducted?

Those are things decided above my pay grade.

EmptyHill1 karma

Why do cities that are near freeways tend to add more and more stop lights that seem to add to the congestion getting onto the freeway from city streets, rather than relieve it? Also how much influence do wealthy citizens in wealthy neighborhoods have on city planners, when it comes to traffic engineering in the way of making their specific neighborhoods have easier/faster access to freeways/city centers? Thanks for your time, I find this very interesting.

gm21 karma

Do you mean the ramp meters? Signals that tell you when you can go up the ramp and onto the freeway? Those are to control the level of service on the freeway facility itself. Basically the theory is that you prefer a higher level of service on the freeway than on the local roadway network, if you have to choose one or the other.

As for wealthy/politically connected citizens - they do have more influence over the traffic system than your average joe. One job I was at, the mayor demanded (and got) a signal at the entrance to his subdivision, even though the warrants weren't really met. Everyone in the traffic department was opposed to it, but political pressure was applied to the department and the order came down from on high to design and build it.

More generally, every city in the world is heavily influenced by the people who develop within it, and developers are typically rich guys or they couldn't afford to make the investments. So yeah, just about every city is designed the way some rich guy(s) thinks it should be. Not necessarily a bad thing, but sometimes it is.

k_stokes1 karma

Are there plans to use cameras / other devices to predict traffic flow, rather than use set timed values?

I can't count the number of times I have been stuck at a red light with numerous other cars, while the cross direction is green with zero traffic flow. After a 10-15 second wait, the opposing direction goes red just before a dozen cars arrive at it and have to stop to let us through.

I can't help but think "if there were a way for the controller to see that the intersection has no traffic flow, and that there are cars which will be arriving from an intersection which is 15 seconds away, perhaps it could have switched this intersection green, let us through, and changed to green in the cross direction in time for the cars which are now waiting", everything would speed up.

gm22 karma

Yeah, there's one system called Rhythm that was deployed in a few places a few years ago. The hype for this thing was incredible, as in it just made traffic congestion almost non-existent. But the reality is that it didn't work as well as advertised, and it has caused some problems and fallen out of favor lately. I'm sorry I don't know the details, but I bet you can find some "lessons learned" type analysis out there somewhere. I might look them up myself, to be honest.

So the answer to your question is that yeah, some attempts have been made to analyze the platoons flowing down the street and time the signals to match, but it hasn't really been done well at this point.

RJFerret1 karma

Have you seen an impact from systems like Google measuring actual travellers by phone location/movements, and are things being designed differently accordingly?

Future impacts of driverless cars? Biggest concerns from your perspective of such?

gm23 karma

Personally, I haven't seen any systems using google data for traffic policy implementation. But maybe that will change - we were recently selected as a consultant for a city around here for signal design, signal timing, and ITS and I know it was based at least partly on our inclusion of a Purdue University professor who's research involves real time performance measures evaluation. Typically that involves installing detectors on the roadway, but maybe we can find a way to use Google's crowdsource data. There would probably be potential for journal articles and more traffic groupies if we could figure that out.

Driverless cars - it would be earthshaking if it went mainstream and widespread, but I'm not expecting it within the remainder of my career (probably about 20 more years.) There are too many legal and policy issues to sort out first, and those kinds of gears grind exceedingly slowly.

Just off the top of my head, how do you get a computerized guidance system to recognize quickly changing roadway conditions (lane closure, tree in the road, etc) the way a driver's brain does? Can it be programmed perfectly? Who is liable when the first inevitable fatality occurs with a driverless vehicle? If we make adjustments to "standard" signal or roadway designs, how does that affect the driving algorithm?

Tourrainette1 karma

Admit it, you really work for the Impossible Missions Force, don't you?

gm24 karma

Your mission, should you choose to accept it, is to drive through this roundabout.


So I live in Irvine CA, and do the traffic lights really change when emergency vehicles are coming through when their sirens are on? I only ask because I was waiting at a light for my turn to go, and then an ambulance rolled through, and the light completely skipped my turn.

gm24 karma

Their sirens don't really have anything to do with it, although they are typically on when the vehicle is "running hot" and preempting signals. Usually they use an infrared emitter that sends a beam to a detector at the signal, which can then let the controller know to give green time to the proper phase of the signal.

There are also GPS based systems which can calculate the arrival time of an emergency vehicle even when it is around the corner and not within line of sight - we've designed a couple of those systems recently.

I don't know what Irvine does, but yeah emergency vehicle preemption is a real thing and it will sometimes cause phases to skip for a couple of minutes while the ambulance rolls through.

justforthis789341 karma

Civil engineering student here! Any illumination design/ITS that's going to impact bicycle traffic down the (heh) road?

gm21 karma

I think there is potential for ITS to be applied to bike routes, but it would take a commitment on the part of the cities to implement it. The main value of ITS is getting information to and from the user - what information might we want to send to the biking public? Maybe street closures, stuff like that.

The challenge is going to be justifying the cost for the infrastructure - the economic incentive of keeping your travelers informed is much smaller for bikers than it is for drivers. But maybe in a big city where there is a lot of commuting and delivering going on with bikes - you might justify some kind of dynamic message sign or maybe an opt-in message delivery to smartphones.

Irruga1 karma

Hi and thank you for all the information so far it was very interesting and inlightening. Where I live there is a left turn in which if there isn't a car waiting with me the light won't change. My question is this, can a light sensor be fixed to detect scooters? Mine isn't the smallest there is but isn't a heavy one either.

gm22 karma

Scooters, and sometimes even motorcycles are a little problematic for detection. There are a few types of detectors currently in common use:

  1. Inductive loops. They are wires cut into the pavement and they look like this. Sometimes they are circular or smaller squares, but in general they are a set of wires that are laid into the pavement. They work when something metal (like a car) is nearby, which changes the induction value in the wire, which can be sensed by the controller as a vehicle.

  2. Video vehicle detection systems (VIVDS) cameras, like this. They can analyze the video to determine when a vehicle is present.

  3. Radar detectors.

All of these technologies struggle to some extent with small profile vehicles like a scooter. Your best bet is to present as wide of a detectable profile as you can. If your detector is a loop, try to park right on top of one of the wires, preferably the middle wire in the quadrapole system shown in the photo. If it's a camera, and you can kind of angle yourself when you're stopped, that gives the camera a wider target to detect.

If none of those work, maybe the detector isn't working at all. I'd recommend that you call and tell them what you told me, and they can make some adjustments. You can increase the sensitivity of the detectors, and if they know there is a problem maybe they can solve it that way.

SilentlyCrying1 karma

Why do some lights seem to last less than 15 seconds? Are they timed based on their location?

gm21 karma

Yes, the signal's cycle length and green time for each approach is designed based upon the amount of traffic present, and they can be changed on a time-of-day or day-of-week basis. So if the expectation is very light traffic at a particular hour, the engineer may have specified only 15 seconds for the maximum green time, at which point it cycles away and serves another approach.

Maybe they are getting more traffic than they expected, and that's why the green time seems inadequate to you. If that's the case, call the traffic engineering office and ask them to take a look at it. Maybe the traffic patterns have changed and they don't know it. They are usually happy to help with a citizen request!

SilentlyCrying2 karma

Good to know thanks! Are there sensors on the road before a light that will affect if it changes to green from red by the car slowing down?

gm21 karma

Yes, if the agency chooses, they can install advanced loops to start the detection process while the vehicles are still approaching. Here's a screenshot from a camera showing an advanced loop: here

The same effect can be done with inductive loops, and radar is designed specifically for this.