When Slower is Faster
One theme that I found myself returning to again and again in Traffic is the often counterintuitive notion that “slower can be faster.” This idea comes up in any number of traffic (and network) situations — ranging from “ramp meters” at highway entrances, to the “speed harmonization” techniques used on motorways like the M25 in England, to individual drivers pacing themselves to avoid “driving into the jam,” to the practice (espoused by hyper-milers and eco-drivers) of not racing needlessly up to red lights, to getting rid of traffic lights and replacing them with roundabouts, which often seem slower but on average process more traffic than conventional intersections. As a study by Australia’s Monash University, titled “The Impact of Lowered Speed Limits in Urban and Metropolitan Areas” notes, even a lower speed limit can paradoxically produce faster traffic (particularly in “medium” congestion). The reason: “A lower speed limit may actually reduce overall travel time by allowing a more harmonic traffic rhythm.”
I was intrigued to come across a lo-fi, but effective, demonstration of this principle on YouTube (where a traffic subculture flourishes), using Martin Treiber’s excellent traffic simulator. As someone who constantly finds himself zipping past drivers at the lights— drivers who had moments before blazed aggressively past me — this is a nice example of how our own driving style helps contributes to traffic problems.
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This entry was posted on Saturday, July 12th, 2008 at 4:43 pm and is filed under Congestion, Drivers, Traffic Engineering, Traffic Wonkery. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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July 14th, 2008 at 10:16 am
I don’t believe this simulation is correct. First the computer model assumes that everyone will queue up equally behind each other in each line when stopped at the traffic light.
I’m sorry, but that doesn’t happen in real life. I can’t tell you how many times I’ve seen 15 cars stopped at a light in the left lane and nobody is in the right lane, or vice-versa.
Also, the trucks get up to speed way too quick in the simulator.
Roundabouts create other traffic situations, traffic lights create planned traffic breaks that allow people to be able to enter a busy road during rush hour, when a roundabout is installed some roads are impossible to enter between roundabouts especially with a left turn.
I went to Canada this weekend and it reminded me how much people don’t pay attention when driving, the customs booth is a perfect example. Drivers will queue up behind other drivers without looking at the arrows to which lane is open. I saw two lines with 5 cars each waiting to clear customs, and two completely open lanes with nobody in them. I go into one of the empty lanes and get to the customs agent, asked him how long it’s been since he had someone come through, he said he was open for 15 minutes and I was the first person to use his lane. By the time he cleared my entry I had three cars behind me and the other open lane still didn’t have a single car in it’s line.
I believe the lemming factor makes many of these traffic predictors inaccurate, you can’t predict rubbernecking, accidents, etc., that lead to really screwed up rush hours. A hot blond walking her dog along a major road will do more to screw up traffic than the one driver that’s driving a bit slower or a bit faster.
I know how fast I have to drive to make the lights, speed limit be damned. If I’m the first car out of the light and nobody is in my way, I’ll make every light without stopping, even if I have to do 10 over the speed limit or go 15 under. Well, that’s in areas where the lights are timed, the sensor based lights – some run on a timer as well and others are internally timed based on the traffic. The best ones are the newest asynchronous lights that use radar to tell if a car is approaching and will change just before you reach it.
-Ron