The Science Of Driving And Traffic - the importance of breaking the rules

Let me state up front that this is not a topic I know anything about, but I have always had a curiosity for it, so let me just throw some thoughts out into the Internets and see if commenters or other bloggers can enlighten me or point me to the most informative sources on the topic. This is really a smorgarsbord of seemingly disparate topics that I always felt had more in common with each other than just the fact that they have something or other to do with traffic. I am trying to put those things together and I hope you can help me (under the fold).

1. Models of Traffic Flow

There are two kinds of people modelling traffic: traffic engineers and physicists. The former use traditional modelling techniques, while the latter indulge themselves in using more esotheric methods, e.g., cellular automata, etc. The traffic engineers, apparently, are not too fond of the models developed by physicists, and I always wondered what the reason for this was and tended to dismiss it as mere professional jealousy and turf-protection. But now I think there is a deeper reason - the two groups do their modelling with two different motivations.

The physicists are testing the math and playing with the computers. Their models are applicable not just to traffic but also to other types of flow, e.g., blood flow. Their main goal is to figure out the conditions that determine when the flow will be smooth, when there will be stop-and-go traffic, and when the whole thing will be ground to a stop.

The traffic engineers, on the other hand, have two goals: smooth traffic flow is one of them, but the other one is to ensure the maximal safety of every individual participant in traffic, something that physics models are barely starting to address. The engineers' goals are practical - how to build roads in a way that maximizes flow AND maximizes safety. For that, their models have to incorporate not just the behavior of the entire system, but also the behaviors of each individual driver based on the real-life behavior of people operating motor vehicles (as well as cyclists and pedestrians). The physics models have only recently made some baby steps into incorporating realistic human behavior into their models. After all, humans beings behave differently in traffic than red blood cells in blood circulation.

I could not find a study that I remember from a few years back that shows that the "jerks", those people we hate because they speed and weave in-and-out of lanes, actually contribute to smooth flow - without them, the traffic would be more likely to get congested. Although the speeders only shave off a couple of minutes of their own travelling time, their behavior prevents the blockage of traffic and thus also shortens the travelling time for everyone else.

For (I think) excellent summaries of the current state of traffic modelling I recommend these two articles: Stop-and-Go Science and The Computer Minds the Commuter. Is there anything better or more recent I should read?

2. City Traffic

Most of the traffic flow models I read about deal with the flow and congestion on highways. I could not find that much on modelling traffic of city streets. Such models must exist, though, as someone must have made some calculations when suggesting roundabouts on Hilllsborough Street in Raleigh (the street that serves as a northern border of the North Carolina State University campus). For the heated debate about this, check out this excellent blog post and comments, this article and these commentaries, just to quickly get up to speed. This discussion has been going on for quite some time now, with quite strong feelings exhibited by the two sides of the issue - the pro and con groups. I have nothing empirical to base my feelings on, but I instinctively aligned myself with the pro-roundabout side. It just felt right. Am I wrong? Why or why not?

3. Car Safety

About a year ago, I have read (in 'Discover' magazine, I believe), several people's essays on the "Future of the Car". Most wrote about new gizmos and gadgets, more entertainment, and more automation. But one was thinking completely out of the box and I loved it! How to improve safety of the cars, he asked? Not by building bigger, harder and stronger cars with more and more nifty safety features - that is just a never-ending arms-race. Instead, take a lesson from the inflatable gas bags - what makes it useful is its softness, not hardness! So, the author argues, why not make the OUTSIDE of the car as soft as a marshmallow? People would still not want to bump into each other because it affects their own speed and direction, but if such a contact occurs, nobody gets hurt! Brilliant!

4. Geography of Driving Philosophy

I learned to drive back in Belgrade when I was about 18 (min. driving age there), but never bought a car so I did not drive there very much. Still, the driver ed there is a long gruelling process, about 40 lessons stretching over several months. During the course, I drove on the highway and in miniscule city streets. I climbed a mountain (and drove back down again on a very narrow twisty road). I drove out in the woods outside the city in freshly fallen deep snow (and my instructor and I helped a couple of other people get their cars out of the ditches). When the city streets were covered with ice one day, my instructor made me go to the hilliest part of town and taught me how to negotiate very steep uphills and downhills on ice.

But, although the driving school was just a couple of blocks away, I had to wait until almost the end of the course until I was allowed (allowed? Forced - I was terrified!) to try to negotiate Slavija (see picture) - a huge roundabout in the very center of Belgrade where there are absolutely no traffic signs! There are some simple basic rules of traffic applicable to the situation, but most of the rules were actually unwritten rules and all the traffic around it was based on driver-driver negotiation. The way people drive there, everywhere in the country, but nowhere felt as palpably as on Slavija, is by such driver-driver negotiations: one part applied psychology, one part hand-and-eye signals.


When I arrived in the States I had to start driving because there is no other way to get from A to B. And that is when I realized that the driving philosophy is different here - it is not based on negotiations, but rather on strict obeyance of much better defined nitty-gritty rules. There are exceptions - driving in Manhattan is more Europen-style in this matter and there may be some other geographical differences with the USA. See this for an example.

There is something about this that makes me uneasy. I have a feeling that many people here drive on 'automatic pilot', lulled into complacence by a naive expectation that strict following of rules will automatically make them safe. I see it in myself. When I have a sense of flow and a full awareness of my surroundings I drive much safer but that also means that I often buck the rules. After all, the rules are just suggestions, the scaffolding on which we build our driving behavior using our knowledge and experience. When we drive we make decisions every moment. Most of the time, the decisions we make will be within the rules and laws of traffic. But sometimes, the best decision is to not follow the rules. Safety is the primary concern. When it is satisfied efficiency comes in as a second concern, followed by wish to minimize wear-and-tear of the car, greater comfort for the driver and passengers, and the fuel efficiency. Blind obedience to rules often does not satisfy either one of these, and when safety is challenged, bucking the rule is the best thing to do.

But, I got a couple of tickets last year for "rollling through a stop sign" so now I obediently stop. And I discovered that this gives me a false sense of security - I do not pay as much attention to what is really happening in traffic around me. That is unsafe!

What I think is happening is that these stop signs are unneccessary - they should not be there - and when I drive well, somewhere deep inside my mind there is a decision to ignore the sign because it is an obstruction in the way of safe driving. They are inside Southern Village - a little urban village that looks like a toy-set for kids. The fact that the streets are paved at all is kinda nice, and that streets have names is probably useful for the mailman, but traffic signs are totally useless and counterproductive because everyone here drives within the "negotiation paradigm" of driving. Unfortunately, there is a police station in the Village and some of the laziest and meanest macho cops find it easy to hide in the narrow curvy streets and quickly gather a bunch of tickets from me and my neighbors without having to go too far. And who is going to argue cognitive science and physics of traffic flow with a guy who so clearly and sadistically enjoys the power of his badge and his gun (and his BLACK uniform - I thought that no force in the world, after the WWII, would deign use black unforms of the SS again!)? Those guys - minority conservatives in town - clearly derive great pleasure in exerting power over their liberal majority neighbors.

So, I was really happy to find that I am not the only one who thinks that most traffic signs are unneccesary or even potentially dangerous. Garry Peterson wrote a great post about this very topic, in which, among else, he quotes from this excellent Wired article:

Hans Monderman is a traffic engineer who hates traffic signs. Oh, he can put up with the well-placed speed limit placard or a dangerous curve warning on a major highway, but Monderman considers most signs to be not only annoying but downright dangerous. To him, they are an admission of failure, a sign - literally - that a road designer somewhere hasn't done his job. "The trouble with traffic engineers is that when there's a problem with a road, they always try to add something," Monderman says. "To my mind, it's much better to remove things."

Riding in his green Saab, we glide into Drachten, a 17th-century village that has grown into a bustling town of more than 40,000. We pass by the performing arts center, and suddenly, there it is: the Intersection. It's the confluence of two busy two-lane roads that handle 20,000 cars a day, plus thousands of bicyclists and pedestrians. Several years ago, Monderman ripped out all the traditional instruments used by traffic engineers to influence driver behavior - traffic lights, road markings, and some pedestrian crossings - and in their place created a roundabout, or traffic circle. The circle is remarkable for what it doesn't contain: signs or signals telling drivers how fast to go, who has the right-of-way, or how to behave. There are no lane markers or curbs separating street and sidewalk, so it's unclear exactly where the car zone ends and the pedestrian zone begins. To an approaching driver, the intersection is utterly ambiguous - and that's the point.

Monderman and I stand in silence by the side of the road a few minutes, watching the stream of motorists, cyclists, and pedestrians make their way through the circle, a giant concrete mixing bowl of transport. Somehow it all works. The drivers slow to gauge the intentions of crossing bicyclists and walkers. Negotiations over right-of-way are made through fleeting eye contact. Remarkably, traffic moves smoothly around the circle with hardly a brake screeching, horn honking, or obscene gesture. "I love it!" Monderman says at last. "Pedestrians and cyclists used to avoid this place, but now, as you see, the cars look out for the cyclists, the cyclists look out for the pedestrians, and everyone looks out for each other. You can't expect traffic signs and street markings to encourage that sort of behavior. You have to build it into the design of the road."

So, I'll quit now, but I'd really like to hear from other people about their thoughts and knowledge about the way human psychology affects driving safety and traffic flow. After all, there must be a reason why robots don't drive cars and humans are needed in the driver's seat - robots would be programmed to strictly obey traffic rules. The fact that they cannot drive cars suggests that rule-breaking by humans is an essential aspect of traffic safety. I hope that CogSci people here, perhaps Dave and Chris, will dig through their literature and enlighten me more on this topic.

Update: Definitely read WHERE THE SIDEWALK ENDS, currently up on Seed Magazine's main page.


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someone needs to teach people how to drive in rural illinois - they are so damn polite! everyone sits at a 4 way stop and stares at each other all trying to out polite the others. Just go! please just be aggressive! ugh.. hahaha...

When I went back to philadelphia last year and got cut off I was actually happy.

"I have a feeling that many people here drive on 'automatic pilot', lulled into complacence by a naive expectation that strict following of rules will automatically make them safe."

You should check out this article on about a thing called "psychological traffic calming." It sounds like a handful of urban planners have come to the same conclusion that you have: that alert drivers are better drivers, and that drivers are more alert when they don't feel complacent. Solution? Oddly, make roadways seem more dangerous.

The article is linked here:

The author mentions a book or two in it for further reading...

When I drove all the way down Broadway in NYC I discovered one of those unwritten rules: if the front of your vehicle is ahead of the front of another vehicle, you have the right of way to change lanes at any time. It worked both ways for me and seemed to be observed almost universally. That would not have worked in any other place I have driven, and would in fact have caused a lot of anger.

I observed an interesting case in Atlanta once. At a busy intersection a group of drivers apparently felt that a red light had stayed on too long, so they started edging into the intersection and eventually assumed the right of way over cars that actually had the green light.

All Indiana and all Wisconsin drivers should STAY OUT OF ILLINOIS!! They usually drive way too slow and are their driving habits are unsafe for them and others. They impede traffic as a general rule, and drive slow in the fast lanes.

ALSO: Drivers that do not notice when a traffic signal turns from red to green, and drivers that drive and talk on their cell phones should be taken out and shot.

Thank you.

A long time ago I read an article about freeway traffic jams, in which the author concluded that on the whole, the congestion is actually made worse by drivers who think they'll get around the jam by changing lanes. The trouble is that many other drivers are thinking the same thing, so the "fast" spots get jammed up by all those drivers changing lanes, and that another group of drivers, watching out for such lane changes, tend to slow down out of caution. I guess it just shows how little we really know about traffic flow.

OTOH, one traffic flow problem has been solved for people waiting in line at, say, a bank or fast-food place. If there are at least three service points, the line moves faster because the people who have a lot of business to do are getting out of the way of all the other people who are there for a quick in-and-out. There's also a psychological advantage in that when the line is more or less constantly moving, people can see that they're actually getting somewhere.

Now, if only the architects of the world would get it through their heads that there's a damned good reason women take over men's rest rooms (the time-honored "guard the door" routine) on occasion. In order to equalize the waiting time in men's and women's rest rooms, they need at least 5 women's stalls for every 3 men's.......

By anomalous4 (not verified) on 13 Dec 2006 #permalink

My perspective is a little different having not owned a car now for going on seven years. But much of the same logic applies. I believe that it is safer riding my bike in the city (Philadelphia) than in the suburbs. Some people look at me like I'm crazy when I say that and respond with "I hate riding a bike in the city: it's too dangerous!" But it's really not.

For one, in the suburbs, a cyclist can get lulled into a sense of complacency by the sparse traffic and large shoulders. Secondly, suburban drivers (at least here in the states) don't know how to deal with cyclists on the road. They oftentimes don't even see the cyclist even though they're looking right at him/her. And also, motorists can drive faster outside of town.

(A few months ago, I read a study where a researcher strapped a motion/proximity detector to himself and went riding with and without a helmet. He discovered that automobiles gave him more room when he was helmetless. I think I saved the url on my computer at home because I was going to blog about that, but then didn't get around to it. This might be a good time to revisit that.)

In the city however, cars motorists are accustomed to dealing with cyclists and so notice them. The cyclist is also more aware of everything around the bike. Bicycling in the city is a constant negotiation of trafficone must always be ready to change trajectory at the drop of a dime. I often hear people complain about "crazy bike messengers" who fly through traffic without paying attention. In fact, it only seems that way to the motorist. I would argue that your typical bike messenger is more cognizant to the flow of traffic than are the motorists who surround him/her.

On point 3,

This principle is already incorporated into automobile design. In some (I'm not sure if its most now) cars, there is a rigid structure around the areas which contain passengers, but the rest of the car is designed to be less rigid. The less rigid parts undergo plastic deformation during a collision. While these parts are deforming, they transmit part of the normal force from the object the car is coliding with to the cage, decelerating the still intact cage (and hopefully an equally intact set of occupants) until the force required for further deformation outside the cage is greater than the force necessary to begin deforming the cage.

I've thought for a long time that most stop lights can safely be replaced with stop signs, and most stop signs can be done away with altogether.

A few months ago, a power failure blacked out all the lights at a busy intersection ("malfunction junction") in my home town. The traffic flowed more smoothly when drivers regulated themselves than when the lights were on.

This year I have to take my daughter to school 5-10 minutes earlier than normal because, since they placed a person to direct traffic the drop-off area is constantly congested and goes far too slow. In previous years, negotiations between drivers (and we are all neighbors and have kids at the same school) made the traffic flow smooth and fast.

Another important point is vision, i.e., the way a person looks while driving.

Focusing on a single point on the road in front of you, or the car in front of you, or shifting the focus from one object to another is potentially dangerous. Instead, one needs to take in the whole 180+ degrees of the visual field without focusing on any particular point. This allows one to see everything - cars in front and behind, parked cars that may want to join the traffic, pedestrians and cyclists, cars approaching from side streets - all of it at once.

I learned this from riding horses - if you look at the next jump, the horse will stop. If you look "at the distant mountains" (that's a trainers' metaphor - really means looking at the entire arena), the horse will fly over the jumps.

During the Northeast blackout of summer 2003, all the traffic lights in NYC went out. Despite the efforts of many "volunteer" traffic cops, many areas became completely gridlocked for many hours.

By PhysioProf (not verified) on 15 Dec 2006 #permalink

Just now discovering your article by way of Dave's piece on bikes breaking/bending road rules. The book TRAFFIC by Tom Vanderbilt is said to address some of these ideas -- I've been wanting to read it but now it's shot to the top of my list.

Driving around New Orleans after Katrina there were many large intersections where the traffic lights remained off or in yellow blinking mode for months. People simply treated the intersections like stop signs, and traffic continued to move smoothly. I got used to pulling up to the intersection, waiting for a one car from each lane of the crossing road to pass, then taking my turn. It actually took much less time to get around, there were no huge columns of traffic at the lights, and no speeding up on yellow lights. I was sad when the lights started working again, and to really drive home the point that the lights weren't about safety, the city next installed red-light cameras.