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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2 Responses to 55 MPH Around Atlanta

  1. Tenner says:

    Highly interesting… I’ve always wondered what would happen if anyone tried that.

    I don’t know the research involved in traffic engineering, but it’s always seemed to be best to let people set their own speeds. Run a road with a token speed limit for a few months, determine the 85th percentile speed, and make up some signs.

    I also personally believe that no speed limit should be set under 30mph. It’s slow enough to stop on a virtual dime, anyone with the sight ability to have qualified for a driver’s license in the first place can see children on sidewalks, and most cars don’t really maintain 20 mph well (especially if you drive a manual).

    Some of these 20mph school zones are unnecessary.

    Of course, I suspect most speed limits aren’t set to be safe but are to make money for local and state police jurisdictions.

  2. Bill says:

    School zones are an entirely different animal. A lot of jurisdicitions have a blanket ordinance that says “during school hours, the school speed limit shall be 20 MPH beneath the posted speed limit” and they implement this without any thought to particular conditions. For example, a kindergarten that has almost no pedestrian or bicycle traffic is treated exactly the same as a middle school in a downtown neighboorhood with numerous pedestrians and a high school with a vast driving-themselves population. The reason is usually because us Traffic Engineers have better things to do than determine the merits of each and every school zone.

    There is a probelm, however, when said school zones are horribly affecting upstream traffic congestion (and there are quite a few of those in the Atlanta metro area). These, IMHO, should have studies, and be more intelligently applied.

    Before I get local officials all over me, however, I will state that I personally believe that several of the aforementioned horribly-traffic-congesting school zones in Cobb County are intelligently applied, but that’s a vast minority of all the ones I’ve ever seen.

    As far as enforcement and revenue is concerned, the police and municipalities don’t really make money doing that sort of thing. Don’t forget they’ve got to pay for the officer’s time, the cruiser, the administration, and all sorts of little things. But, let me also reiterate that a speed limit that isn’t obeyed without enforcement should be raised.

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