Sunday, October 26, 2014

Driving faster can get you there more slowly

When you drive faster, you need to maintain more space between cars for safety. In Atlanta, where drivers tailgate at high speeds and in the rain, accidents are common, which blocks the road and slows down the traffic.

By Bob Dallas
October 20th, 2014 (Oct. 21, 2014 in print)

The times are a-‘changing, and so are the speed limits on the Perimeter (I-285). The Perimeter speed limits on the south side running below I-20 were raised to 65 mph several months ago. The Perimeter “Top End,” however — not so fast. There, the congestion, number of exits and safety concerns were significantly more impactful. While conditions on weekends and late nights supporting shifting the speed limit from 55 to 65 mph, peak hours suggested downshifting to a much lower speed.Before we protest too much, lowering the speed limit pre-peak is the most effective way to improve performance by averting the worst congestion during peak drive times.Why does this work? It avoids the accordion effect that creates stop-and-go traffic and significantly reduces the number of severe crashes that degrade the flow. Studies consistently show the optimal speed that accommodates the most vehicles is just under 45 mph. [ I read about studies showing this in print, but could never find it on-line. Surely there somewhere, in the 100,000 results.]


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