1) Is it faster to weave or to just stay in one lane (if so, which lane)?

Is it faster to go faster? Yes. If you meant 'it seems like I'm stuck behind all these cars and that lane is going faster.' yes, it does seem like it. Page 40 and on of Traffic: (preview available on amazon, great book for any reader btw) does a pretty good job of explaining it. (other note, the USA will never have countdown timers on traffic signals. Edit for clarity: Pedestrian timers are ok, vehicle timers not so much...) tl;dr Queueing theory is hard and it's all in your head.

2) Are left hand turns really that dangerous? Why?

Yes, left hand turns are more dangerous. Imagine cars running randomly though an intersection. OK, maybe you don't have to imagine that. A diagram will show that a car that turns left has many more points of intersection than a right turning one. Each of those dots represents a potential point of collision. Once you start adding more lanes you will have many more points of possible collision. Add in geometry and vehicle shadowing on opposing left hand turns and it makes left hand turns even more dangerous.

3) Would we really be safer if the yellow light was extended for a second?

You wouldn't be safer (assuming that the yellow time is currently correctly timed.) The yellow time is supposed to be timed based on this formula This works out to roughly: 25 MPH -- 3.0 Seconds 30 MPH -- 3.5 Seconds 35 MPH -- 4.0 Seconds 40 MPH -- 4.5 Seconds 45 MPH -- 5.0 Seconds 50 MPH -- 5.5 Seconds 55 MPH -- 6.0 Seconds

If it's a different time, it is probably wrong. The point of the yellow light is to make sure that if you are well ahead of the light that you have time to stop before you get to the stop bar. the point of the red light time is to make sure that if you didn't have time to stop for the intersection that you will clear through the intersection before the green time start on the perpendicular streets. That point where you don't know if you should stop or go? The DILEMMA ZONE.

EDIT: adding more yellow time would additionally be dangerous as people would start to game it and count on it as green time.

4) What causes the seeming random slowdowns on highways?

Call it the butterfly effect. When there is a lot of traffic, a SHOCKWAVE ripples backwards at a certain speed based on traffic speed and reaction time of drivers. The larger ones which involve stops may be caused by a collision well ahead of where you are, but by the time that you get there, everything is gone. A stopped vehicle will reduce the roads capacity to handle that flow of vehicles and will back things up until it is removed. Think of it as putting a hose into a watering can where water comes out the spout just as fast as you are putting it in, a normal traffic scenario. Now, let's take your hand and block a number of the holes at the spout. Water will back up in the can until the holes are unplugged. Now based on the capacity of the spout will traffic clear after it has been backed up.

5) Are toll roads (i.e. turnpikes) designed any better than "free" ones, or are they just faster because there are less people?

Depends on what you mean by better. Turnpikes and interstates are all designed to a certain standard, which in almost every case (USA) is from AASHTO's Green Book. They are in general better taken care of in maintenance based on their steady funding from the tolls, whereas other state roads have to compete with all other state roads for limited funding.

UPDATE: Trafficwaves is a pretty frequent question. I subscribe to this mindset. Slamming on the gas and then brakes is no way to go through life in traffic. give that next guy some room, if someone fills in that gap, give them some room too. maintain a speed in congestion.

UPDATE2: the dutch and their no traffic signals. i guess it works for them in their low volume villages, but im not to hopeful for high volume roads and in 'Murica in general.

UPDATEFINAL: i never thought i would ever get to the front page. lurking is more my thing. lots of fun, and if there was some question buried that i never answered, try a pm. thanks,

Comments: 2803 • Responses: 41  • Date: 

jeronimoe617 karma

I have a theory on how to fix the random slowdowns on highways, please suggest this to your superiors...

What if we just line the highway with giant speakers, and when traffic backs up for no reason, someone gets on a microphone, and blasts over the speakers, "1, 2, 3, GO!". Everyone accelerates at that time, and voila, traffic is gone.

Another idea I had was to build a bridge right next to the Delaware Memorial Bridge that costs 25 cents less, and when you get on the bridge and pay your toll, you can order McDonald's, when you are at the end of the bridge, you pick up your food.

Fucking Genius... if you want to hire me, send me a PM.

alwaysready305 karma


Rape_Van_Winkle77 karma

Here is my idea and yes I know there is probably an entire field of research papers I haven't bothered to look up, but, pressure sensitive brake lights. If cars didn't have a simple on or off brake light, bit conveyed more information like tapping on brakes or slamming by having a triangle shape of rows of lights that light up based on how hard the brakes were being pushed. Anybody have any research onthis?

IxKilledxKenny22 karma

And then we could customize our lights to look like loading bars!

alwaysready58 karma

The newest technology of loading bars is a spinning circle. We don't need some sort of hypnotoad like effect out there. Think of the children...

Peaced214 karma

Did you study fluids mechanics ?

alwaysready580 karma

Yes, but not very hard.

[deleted]167 karma

What is your opinion on traffic circles? Why aren't there more of them? They seem to be much more efficient than 4 way stops.

Also, do small towns use stop signs to regulate speed? It feels like when I go through a small town there are stop signs everywhere.

EDIT: I meant roundabout. Just learned the difference between the two.

alwaysready294 karma

Traffic circles are garbage. Roundabouts are great. I think this covers the difference, though i didn't really look at it.

Small towns think stop signs are safer. If you have to stop, then you won't be running over their children.

Wommie180 karma

After living in the UK all my life until last summer when i moved to the states. It really annoys me spending so much time stood still at traffic lights, when you could put a roundabout there and keep the flow of traffic moving much smoother. Of the very few roundabouts I've seen the locals complain about them bitterly. Is it really that hard to open your eyes whilst driving and not be spoon fed by traffic lights?

Lepsis212 karma

I can tell you from experience (American with a new roundabout in our college town) that the frustration comes from 90% of the population having 0 clue what the fuck to do in a roundabout leading to more accidents.

Vogon_Poet43 karma

"leading to more accidents"

That's kind of the beauty of a roundabout though. IF there is an accident, it's rarely devastating because a roundabout all but eliminates the probability of a head on collision (most deadly). Also, they're safer for pedestrians because they only need to pay notice to one direction of traffic.

Also, although I haven't got any statistics to back me up, I do remember hearing that overall accident rates in roundabouts are far lower than traditional intersections.

alwaysready130 karma

As it turns out, i do have the statistics in a pamphlet in front of me.

  • 90% reduction in fatalities
  • 76% reduction in injuries
  • 35% reduction in all crashes
  • no data, but slower speeds as found in roundabouts are better for pedestrians. *offer does not apply to blind pedestrians.

RabidOctopus38 karma

Who the hell thought it would be smart to give priority to traffic entering a traffic circle? If terrorists infiltrated all our DOT's, they would use traffic circles to kill Americans.

alwaysready17 karma

Only the first few every couple hours depending on emergency and wrecker response.

RabidOctopus159 karma

Will we ever have bicycle highways?

alwaysready157 karma


RabidOctopus22 karma

Have you ever designed anything around bicycles?

alwaysready25 karma

bicyclestatue.jpg ?

knumbknuts143 karma

Why the fuck, if I accelerate normally and drive the speed limit, or slightly below it, will I get stopped about every third light, but...

if I drive 10 - 15 miles per hour over the speed limit, I will hit all the lights?

I have noticed this is in a lot of yuppie enclaves in Orange County, like Irvine.

alwaysready143 karma

Trafic signal coordination. in your case, here. You have to get yourself in the green band. Take a look at this time space diagram representing you driving down the road in a coordinated corridor. The lights are timed to turn green just in front of you, and even better if you just hit (not literally) the back of the queue in front of you. if you start driving too fast, your trajectory will be flatter and you will go outside the green band and will hit the red light. *a straight line at the same angle as the arrows represents you traveling the speed limit. as long as you maintain that speed/angle, you will 'miss' the red lights. Go faster or slower and you will hit a red/wall.

knumbknuts71 karma

Yea, I'm aware the concept, but they're doing it wrong. It's timed for 10-15 mph above the speed limit.

In 1985, I could drive at 32 mph all the way from one side of Fresno to the other (before they started putting in pressure plates). The speed limit was 40.

I've tried speeds at the speed limit to 10 mph below and it doesn't work. It's clearly timed to 10 mph above. I notice this in Carlsbad, CA, also.

What you are describing is the exact opposite of my experience (if above the speed limit is considered "too fast")

alwaysready93 karma

I read that wrong. And they are doing it wrong. You could complain, but maybe the faster speed is working for you? In times of congestion, it will make everything worse.

ethomp24 karma

Now I want to be a traffic engineer. You guys have so many cool diagrams.

alwaysready30 karma

Prepare to have your mind blown.

Lucavious113 karma

What's the best thing to do when you find yourself in the dilemma zone? Usually my response is to forget everything I know about driving and either stop way too fast or run a red light.

alwaysready120 karma

It depends. If you have some guy riding your butt, you probably want to run that yellow.

EDIT: yeah, didn't even plan that one...

jolsh83 karma

How does one go about becoming a traffic engineer?

alwaysready147 karma

You'll need a 4 year degree in Civil Engineering, with lots of transportation classes.

vinfx78 karma

  1. Why does speeding over speed bumps seem like they impact the car less than driving over them at a slower rate?

  2. What's your opinion of the diverging diamond? What's the outlook for those?

  3. What does speed enforced by aircraft mean? Is it bullshit?

  4. What's more interesting to build? Bridges, tunnels, highways? Fresh construction or upgrades?

alwaysready73 karma

'3. It's true, but odds are they aren't going to having it fly all the time. You probably have big rectangles of white on the side of the highway. They time how long it takes you to get to the next one and then radio down for the police to nail you.

vinfx46 karma

It seems like the fuel consumed to keep the helicopter in the air all day, the pilot's wage, and the general cost of aircraft upkeep would exceed the revenue brought in for the speeding fines.

Especially if you consider they could hide on the other side of a bridge or build small lookout towers (disguised) along the highway and achieve the same effect.

alwaysready97 karma

If you aren't in it to make money and are indeed wanting people to slow down and not kill themselves, it could make sense?

tek001156 karma

Is it really better/faster to drive as far up as possible when coming to a lane closing?

ie "the zipper effect"?

alwaysready125 karma

I hate those people, and I *was an active blocker. But yes, I believe that those jerks make better use of the roads capacity and better for us all. Safer? Doubt it.

EDIT: yes, it is best to merge late.

kyuubi4228 karma

wait... you admit that they're doing the right thing, but you hate them and make it harder for them to do so...?

alwaysready71 karma

part of the psychology of not liking to be cut in line. :)

noirthesable55 karma

Do you happen to know why it always seems like there's some giant traffic-based conspiracy against you when you're running late for work, or is that question better suited for a psychologist?

alwaysready157 karma

It always runs *well for me. Must be just you.

TheLoveTin49 karma

"the USA will never have countdown timers on traffic signals"

NYC already has countdown timers at intersections. They're for pedestrians, but just as useful for drivers.

alwaysready77 karma

I should have been more specific. Pedestrian countdown timers are required for new and rehab jobs. You won't see any for vehicles.

Ovedya201148 karma

Here's one that I was actually wondering the other day: I work downtown, and travel down the main artery to get there. As I approach downtown the blocks are smaller and each on has a signal. I noticed the other day that the signals don't appear to be in sequence with one another. They don't allow "bunches" of vehicles to go down the road smoothly. The light at which you are stopped will turn green while the one just beyond that will be cycling through the yellow. This causes traffic to stop at each light. Occasionally the light beyond will turn green while you're still stopped at the red. So my question is do you program the lights down main arteries in sequence or do they each run independently of one another depending on traffic? I though that at rush hour aignals were aupposed to "prefer" traffic running down the busier main arterial streets.

alwaysready113 karma

In your particular case, it appears that your traffic signals have not been coordinated. It can be expensive to tie them together so not just everyone goes out and does it. I would recommend complaining to people who would refer to you as a constituent.

baaron48 karma

When you sit down to create traffic patterns for a large city, say something like Lower Manhattan, is there a design application you would use? Is there something that takes the drawing/diagram from this application and spits out values to key into the different traffic control boxes around the grid or would this be controlled by a central computer?

alwaysready66 karma

Synchro/ Simtraffic is the *USA's choice. But other models like corsim and hydra do it different. They each do something better than the others, but Synchro seems to have usability down pretty good. **edited for USA! USA! USA!

lars45647 karma

Can an intersection be converted to a roundabout?

alwaysready164 karma

Anything can happen, with IMAGINATION. Or in this case, with money and right of way. Which is money. Assuming that this is a 1 or 2 lane in each direction roadway, it (based on many different circumstances, your scenario may differ) it should be a roundabout. Once you start going above 2 lanes, you're going back to the dreaded rotary or traffic circle, and 'HEY, there's Big Ben, kids!'

woahmanitsme33 karma

Do you think that round-abouts are actually way more efficient than four way stops? I live in Toronto and just outside out city there are a whole bunch of them, but I'm not convinced on how much time they save (after all the confusion of people doing them wrong)

alwaysready90 karma

For most situations, they are definitely better. Psychologically at least they are better. I go when there is space for me. Not when the monkey in the traffic signal box punches the button for my lane.

FlexorCarpiUlnaris12 karma

I thought that you were the monkey in the traffic box. Isn't that what this AMA is about?

alwaysready49 karma

I'm on break.

Vassosman32 karma

This may not be your field, but is it true that you dont have to stop for a stop sign that are not put in by the government? Picture

alwaysready86 karma

You should probably stop. I'll let you work that out with your lawyer though.

TheFAJ30 karma

Did you go to school with the intent of traffic engineering? What is your degree in?

I am curious, as I am a CE that is in a different field than I went to school specifically for.

alwaysready97 karma

I wanted to build bridges, but I missed a static analysis class once. :O

thefirebuilds24 karma

Is it true that bridge weight limits are determined by driving larger and larger trucks over the bridge, until it fails, and is subsequently rebuilt? Bill Waterson taught me that.

alwaysready27 karma

True Story, but now they can mass produce bridges and only have to destroy one bridge before they know what is good for that lot.

IdiothequeAnthem30 karma

Is there any awesome improvement you could make to our road system that is impossible to implement due to politics?

alwaysready144 karma

Every improvement to make things better is hindered by politics.

shaggorama24 karma

I understand that certain technologies are in place on some roads to conditionally trigger the light to change. For example, magnets in the road that can tell if people are waiting at the light, or microphones that recognize approaching sirens. What I've always wondered is: if all of the lights in an area are designed to operate in coordination, does that mean these technologies affect the traffic pattern of several lights, or just the one?

alwaysready36 karma

I've never heard of microphones that sense sirens, but certainly strobe lights. Two concepts, the first being traffic actuation where the signal detects cars and wants to let them go. In a coordinated system, the main road will get lots of time, and use it all whether there are cars there or not. The signal will then allow the minor road some time or skip it if it doesn't see anyone there. not all the time, not every scenario, every light is timed and programmed differently.

For the other technologies that interrupt the traffic, it will take several 'cycles' of that particular signal to work out the delay and interruptions until things run smooth again.

hahagutlaugh20 karma

How long do you estimate until cars can drive themselves?

alwaysready50 karma

I thought darpa already had them worked out pretty good. But for the mainstream? For when all the old cars are off the road and we only have self driving cars? IDK, but Sarah Connor had some misgivings.

elkta19 karma

Pedestrian question: how are those "push to cross" buttons actually supposed to work? Because I'm pretty sure, they're there just to fool me into thinking someone cares about me crossing the road safely.

Also, how can I tell when one is not working properly? (So, I can write some nice letters as a constituent.)

alwaysready17 karma

push button. receive bacon. signal says there is someone there. push it just once. When traffic isn't driving straight at you, you will get the cross sign and enough time to make it across assuming you travel at 3.3 ft/sec or 4.0 ft/sec if you are in an area of a lot of seasoned citizens.

sirmonko19 karma

  • if a genie gave you one wish you could only use to influence driver behavior, what would it be? (something like "no texting while driving" or "keep the proper distance to the car in front" ...)

  • what are the (near) future technologies you're excited about? (e.g. darpa grand challenge, ad-hoc car to car networks, stuff like that)

  • if you had a ton of money to realize one project of your choice, what would it be? (traffic related. no giant massive gold penis statues!)

alwaysready20 karma

talking on a phone and texting especially is more dangerous than driving drunk. If you talk or text while driving, do the other road users a favor and instead just get loaded. OBVIOUS EXAGGERATION BUT NOT REALLY THAT MUCH Because at least when you are on the phone you can hang up and go back to whatever normal is for you.

karmadogma18 karma

  • Do you think dedicated lanes for buses, car pool, etc. are a good idea or does that just mean one less lane for everyone else?

  • Do you think trucks in the US should use all the same roads as cars or should there be more restrictions like they have in other countries?

  • Should the speed limits be raised? I know its an unwritten rule that everyone goes 5-10 mph over anyway but what if we actually bumped up the speed limits and then required people to stay below them so they were actual limits.

  • I've heard that slowly rolling up to a red light will trick it in to thinking there is a long line of traffic and make it go green? Urban legend or true?


alwaysready17 karma

The first two is more on driver behavior. You want to influence people to do something, make it easier for them. Do you want them to take the bus or carpool? Give them their own lane to do it in/ make their commute much quicker. Assuming that there are mostly enough lanes anyways, it could make things work better.

efarther16 karma

Ok, I've got one for you.

Here in the center of hell I call Tyler, TX they have installed flashing yellow arrow lights for left turn lanes. A couple of scenarios I've seen:

Red Flashing yellow arrow Red

Red Flashing yellow arrow Green circle Yellow/Yellow flashing arrow Red

Red Flashing yellow arrow Red Green Red

My take, and the common belief, is that the arrow means you have the explicit right-of-way and that yellow means the light is about to change to red so use caution. The problem is that the oncoming traffic has a green circle so they do not have to stop. There've already been several accidents and one death (someone from out of town) due to these asinine lights.

What's your experience with these stupid arrows?

alwaysready14 karma

They were recently allowed under the MUTCD you can't do the sign or signal unless it is in that book. I'll let Oregon talk you into them.

lomeon14 karma

It seems to me that the best way we could improve traffic is through driver education. I live in Atlanta, and I'm fairly sure that a majority of the drivers have a very limited understanding of the rules of the road.

Is there anything being done or that could be done about this?


other note, the USA will never have countdown timers on traffic signals.

Why not? Can you please elaborate on this?

alwaysready9 karma

Re: countdown traffic timers. They don't have a positive effect on the roadway or driver safety. FHWA won't even look at them as a test case study.

SellYouOnAnything14 karma

Two questions:

  1. What kind of social manipulations would you want to implement on highways and roads to speed up traffic, but can't because its impractical/annoying/unethical?

  2. Help me settle a dispute. Traffic heading four directions meets at an roundabout. Each in-road is two lanes, one for each direction. There are stop signs at each road. Is that how its supposed to work? Whats the deal with the stop signs?

alwaysready59 karma

  1. unethical?

  2. If it has stop signs, it isn't a roundabout.

SoupdupGent12 karma

Have you done any studies on differing systems? Do any countries have noticeably better traffic systems than the average?

alwaysready27 karma

I have for my own. I haven't been abroad or looked a lot, but generally the europeans have been doing it longer and have different/better ways. But they don't drive like americans...

krisssy11 karma

On traffic lights, why does a yellow light show before the green light? What would be wrong with a direct red -> green change?

Edit: This is in the UK and Europe. I didn't know it was such an unusual sequence. To clarify, this is the exact sequence: http://i.imgur.com/N9CwL.png

alwaysready23 karma

See the beginning. You need time to make sure that there is no one there to crash into.

negatroyd11 karma

What's your opinion of congestion charges (such as the one in place in London)?

alwaysready24 karma

Paying money seems to be a pretty effective way of changing behavior. I think studies have shown that. Paying a large fine on a speeding ticket makes people slow down longer than other punishments. Congestion too high? I will discourage and profit from it by charging you to be a part of it.

immatureboi10 karma

Do you think there are more ingenious ways to go about urban cycling? I mean, currently, despite the increase of bike lanes there are still cycling related casualties.

also what are your thoughts on applying chaos theory on traffic?

alwaysready15 karma

Engineering for cycling traffic has a lot of room for growth. As for chaos theory, I don't necessarily want Isla Nublar around my town... yet.

RabidOctopus7 karma

When I drive in Sweden, the lights seem to treat me like a celebrity, changing green before I even arrive. In the USA I frequently sit there yelling that no one is on the side street and the light should have changed seconds ago. Why can't we have traffic lights that react quickly to traffic?

alwaysready29 karma

We do here and there. I'm going to guess that Sweden has more money to go around for less miles of highway. The gas tax hasn't been raised in the US in a LOOOOooooOOOONGcat time.