55 MPH Around Atlanta

This link takes you to the google-hosted video of an exercise conducted by some students. They examined the effect on I-285 around Atlanta when everyone is forced to drive the legal speed limit (55 MPH).

Some background for anyone who doesn’t live or drive around Atlanta: I-285 encompasses the entirety of the downtown metropolitan area, plus all of the inner subburbs. It’s about sixty miles in circumference, and all of it is signed at 55 MPH (I think—not too familiar with the east side), by ordinance, I assume. 55 MPH on I-285 is rather slow in freeflow conditions. Generally the median speed is around 60 MPH and the 85th percentile speed is probably around 65 MPH (let me emphasize that these are personal estimates. I have not done any speed studies). So, 55 MPH is going to be, outside of congested conditions, around the 20th or 25th percentile; 80% of people will want to travel faster if given their druthers.

What does this all boil down to? Watch the video and see. It’s impressive.

Of course, if you do a Google search on this thing and read some of the comments, you will find a large number of people who think that this was just a stupid stupid dangerous prank that proved nothing. Others think that this shows the uselessness of a speed limit that is set too low and not enforced.

My opinion goes with the latter people. It is silly to have a speed limit which is artifically low and not enforced. Several studies (here, and here, there are others) demonstrate no increase in crashes or crash severity by increasing the speed limit to the 85th percentile speed. These same studies show that reducing the speed limit has no actual effect on drivers’ preferred travel speeds. Agressive enforcement will cause drivers to adhere to a low speed limit, but the effect has no lasting duration. As soon as the enforcement is gone, drivers will return to their previous practice.

Anyhow. The video is cool. Have a look.

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