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Going With The Flow

Traffic sucks. It wastes time, it adds to pollution, and it increases driver stress levels. While taking public transit can be a good alternative, often that option simply does not exist. Making traffic worse are those all-too-frequent episodes when, after crawling along for an hour, the traffic suddenly -- and inexplicably -- picks up, as if the traffic jam was nothing but the ghost sensation left over after the original trigger had long ago departed.

To the surprise of some, more roads and more lanes don't help. Traffic jams don't occur due to the number of cars on a given road so much as due to the distance between cars. Less space between your car and the vehicle in front of you means that you have less time to react to sudden moves, and are more likely to engage in a kind of high-speed stop-and-go, hitting the brakes briefly in response to the car in front of you doing so; if the car behind you is driving too close to you, then it will also have to brake, and the too-close car behind it, and so forth. These "pinch effects" propagate backwards along the highway like a wave.

The simplest solution is for people to drive more intelligently, keeping sufficient space between vehicles to buffer the transient braking, sudden lane changes, and unexpected (but brief) changes in speed of the car in front. You're also less likely to end up in an accident if you leave more space. But since traffic planners and safety experts have been trying to get people drive this way for a long while without much success, it's a good idea to look at some technological assistance that might help.

New Scientist and The Economist this last week identified two very different technological approaches to reducing the driver-distance traffic jam problem.

New Scientist looked at a German traffic simulation system used to predict where these "pinch effect" traffic tie-ups will occur on the autobahn. Recent changes to the model, taking into account the fact that cars can't slow down instantly and the bad driving habit of keeping too close to the car in front, allow the model to "see" incipient jams up to an hour before they form. Once predicted in this way, the information can be made available to drivers, who can then change their driving routes or times accordingly. Unfortunately, the system is currently a victim of its own success: so many people choose alternate routes based on the predictions that the forecasts are becoming less accurate.

The Economist, conversely, is looking at "Adaptive Cruise Control" (ACC), which combines standard cruise control speed management with vehicle radar watching how close the car gets to the vehicle in front. According to projections by the University of Michigan, if 20% of the cars on the road were equipped with ACC, the clear-highway traffic jams would be eliminated (this suggest, of course, that a similar result would obtain if 20% of human drivers drove better, but I digress). This sounds great, except that the system isn't smart enough to adapt the way human drivers do, and ACC can actually make things worse under certain (unspecified in the article) bottleneck conditions. Ironically, the solution suggested by the developers is to let ACC vehicles driver closer the car in front than would otherwise be safe; since ACC systems can react far faster than humans to sudden changes in condition, even vehicle distance of less than a second between cars can be safely maintained. The article doesn't mention what happens when the ACC computer fails.

So which will work better -- more information or more computer control? From a just-in-time, flexibility perspective, the individual car ACC system is the winner, making traffic jams less likely regardless of the path or time chosen, although if too few drivers have the system (or drive safely), the effect is minimal, and the ACC-equipped car is stuck. From a plan-ahead/plan-for-trouble perspective, the road information approach is better, as it makes it possible (in principle) to avoid the tie-ups completely regardless of what you're driving, and if the computer system crashes (as they all do), the worst that happens is that you're in an unpredicted traffic jam.

Fortunately, both approaches are complementary, and are moving from the labs to the real-world. The ultimate effect of these developments may well be that traffic tie-ups based on too-close driving will be a thing of the past sooner than we think. Quite a pleasant surprise.

Comments (6)


computer controls be damned! ride yr bike! solution to the indemic problems of traffic in my city is to ride through all weather and break the law constantly. The cops have better things to worry about, at least in minneapolis, and every sweaty motorist that sees you gliding by at an empty red light gets just a little more push towards actually getting off his ass for once.

. . .if the streets were taken up by trucks hauling cargo and cars driving over 30-40 miles, we wouldn't have a traffic problem. we wouldn't have an obesity problem either, but i digress.

you posted an article earlier about some scandinavians with the postulate that city traffic is regulated mostly by legal and illegal conditions: people can't go a certain speed, people have to stop at stop lights, people have to allow other cars on the freeway, people tailgate other people instead of going around them because they'd have to break the speed limit.

Illegal action when it is difficult to enforce (ie bikes) is useful, direct, and a whole assload of fun.


The implication that when everybody leaves more room then traffic would flow more smoothly is incorrect. One of the main reasons that standing waves are created in heavy motorway traffic is because people leave proportionately more room at higher speeds than at lower speeds. Generally, people feel quite comfortable leaving just a few meters at, say, 50km/h, but leave three or four times as much space at double that speed. Therefore, as a strech of road can carry a given number of cars per minute at low speed, the amount of road each car takes increases faster than the speed as cars accelerate and so the road capacity decreases slowing everyone down again.

Solutions are difficult. Forcing slower speed traffic to drive further apart is effectively throttling the throughput. Encouraging people to drive closer together at high speed doesn't sound too hot, either, but the new distance sensors might help. I still remember the effect of anti-lock brakes when they were first common and the tendency for some drivers to drive faster and leave breaking until much later - ugh.



I just got back from Houston, where they have an interesting solution to the problem of traffic congestion. It's called 'quitcher whinin', you tree-huggin' liberal!'

Granted, the guy I was talking to said it in jest, but I think he was, as Al Franken says, joking on the square.

In congestion, I drive the average speed (that is, neither stop nor go), and try to alter speed as gently as possible. I let as many people on at on-ramps as I can, and it's fun to watch them try to figure out my motivation. Which is: The merge is the most alarming part of the freeway experience, and people either slow down or get frustrated when they're alarmed. I want to avoid frustrated, alarmed people, so I don't frustrate or alarm them.

Just a pleasant confusion. :-)


There are 3 main problems with bublic transit and roads.

1 Politicians are idiots and want you to go exactly where you dont want to be through places you dont want to see via methods you dont want to use. For mass transit this means to get from here to there you have to go through 52 places that quite frankly no one on earth wants to be in or see and you have to do it at the wrong time to boot. For roads this means the one place you want to get to is 52 lane changes away most of wich you cant make unless its 3am on a thuesday night and its snowing... in which case you can make the lane change but cant get off the freeway anyway...

2 Poeple are creepy specialy when you cant get away from them for extended periods of time.. aka traffic jams and mass transit.

3 No matter where you go you cant get there from here unless there is where you realy dont want to get to and here is where you realy wanted to stay anyway.



Unfortunately, your driving style could get you into trouble in some places. Here in Europe, the rule is often one-one filter merge at congested on-ramps. Letting two on is likely to get you rear-ended by a surprised local and a ticket for dangerous driving. Local law enforcement have lots of discretion apportioning blame at accidents and complaining will just get a local equivalent of: 'quitcher whinin', ...!'.

(100% jest. Who's Al Franken?)


BTW. The British English (northern edition) equivalent of Quicher whinin' is "On yer bike". Quite appropriate really.


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