Friday, November 7, 2008

Seeing red, part 2: timed lights

On several one-way streets in downtown Louisville, traffic signals are timed to allow vehicles to proceed at a steady speed through a string of intersections without needing to stop for any red lights. These synchronized traffic signals work well as long as the vehicles can maintain that set speed within a few mph.

In my experience, I can ride through 6 or 7 green lights in a row on some of these streets if I ride fast: at least 23 mph. On my commuting bicycle, I simply can't ride that fast. Yesterday, I did some measurements and calculations to learn how fast a bicyclist needs to ride to keep up with the traffic signals, and if any slower-than-car speed could allow a bicyclist to cruise through all green lights.

For the test case I used East Main Street, a common commuting route with a bike lane. The signals are timed to allow a driver going at 34 mph to go from a green light at one intersection to green lights at all of the following intersections. Riding at 24 mph, one would fall behind the signals by about 5 seconds each block plus another 5-10 seconds delay if you need to start from a full stop or a low speed. This fast rider would get through about 7 intersections before getting stopped by a red light. At 20 mph, a rider would get through about 5 intersections before encountering a red light; at 15 mph, 3 intersections; at 12 mph, only 2 intersections before needing to stop again. How slowly would you need to ride to encounter all green lights? You would need to poke along at less than 10 mph.

Someone riding from the east end of Market Street to 3rd Street, for example, would have red lights add 1 minute to the trip if she or he rode at 20 mph. Red lights would add 2-1/2 minutes to the travel time for a 12-mph bicyclist. That comes to a 30% time penalty for the 20-mph rider, and a 40% time penalty for the 12-mph rider.

If Metro re-synchronized the signals for a 25 mph speed limit (instead of 35 mph), the 20 mph bicyclist could get to 3rd Street or beyond without stopping, and the 12 mph bicyclist would arrive one minute sooner. But what about the motorists who could drive at only 24 (instead of 34) mph? The travel time difference for this 10-block trip would be 51 seconds.

Bottom line: If Louisville (or another city with synchronized downtown traffic signals) wants to make its downtown safer and more accessible to bicyclists and pedestrians, they need only re-time the traffic signals for a lower speed. This will decrease the number and severity of car crashes (including crashes with bicyclists and pedestrians), make it much easier for pedestrians to cross the street, and allow bicyclists much less delay due to red lights. The cost - a minor inconvenience to motorists.

2 comments:

crankedmag said...

Minor indeed! I completely support the idea of slowing the traffic down in this town to a more reasonable 25mph. I've been noticing that many of the routes I take from Old Louisville to the Clifton neighborhood are rated at 35mph; some of these routes are what are labeled "signed bike routes" too!

35mph is too fast for neighborhoods and downtown. 35mph, as most can agree is not too much slower than 40mph. Like driving on the freeway, I remember, when it's posted 65mph, "what's the problem with driving 70mph?" Speed limits always get pushed—and 35mph is too too fast!

Great post, and great research.

crankedmag said...

Just came across this tonight:

http://www.streetsblog.org/2008/11/11/shocker-speed-limits-are-useless-without-enforcement/