Fri04182014

Speed limits: A lesson in physics: Other Voices

Note: The Los Altos City Council voted Aug. 20 to retain the city’s current speed limits. The following piece was written before that action, but the subject matter remains relevant – thus its inclusion here.

It makes sense for the city of Los Altos to periodically reconsider speed limits on local streets. I applaud the use of the extensive data generated by the recent traffic survey. However, using data to make traffic safety decisions without considering the laws of physics can be dangerous.

The distance a car travels from the moment the driver decides to brake to the point of stopping is not determined by a simple ratio involving the speed. The car travels some distance while the driver is reacting to the obstacle in the road, before the brakes are applied. The car travels an additional distance while braking that depends on the square of the initial velocity. The combination of these two distances is the stopping distance.

Using typical numbers, a car traveling 30 mph will have a stopping distance of 109 feet. If the speed of the car is increased by 17 percent to 35 mph, the stopping distance increases to 136 feet, a 25 percent increase. The proposed 40 percent increase in the speed limit on Grant Road between Homestead Road and Grant results in a 60 percent increase in stopping distance. This nonlinear relationship between speed and stopping distance is not intuitive for drivers. It results in a dangerous hazard for pedestrians, cyclists, wayward animals and other drivers on residential streets.

Physics also sheds light on the relationship between vehicle speed and pedestrian deaths. The energy of the collision depends on the square of the speed. This explains why 5 percent of pedestrians struck by a car going 20 mph are killed, while 45 percent struck by a car going 30 mph are killed. Increasing speed increases the likelihood of collisions with pedestrians and the chances of a fatality from the collision.

I understand the reasoning behind using the average speed of traffic to set speed limits. It prevents the creation of speed traps that can punish safe drivers. However, the residential character of the streets being considered for an increase should trump the convenience of the drivers using them.

If the current speed limits had been enforced and traffic-calming measures implemented prior to the survey, the average speeds would have been lower. We should not allow scofflaws that value a few seconds of their own time more than the safety of children crossing the street to set the speed limit.

The city of Los Altos has a problem regarding pedestrian safety. According to the California Office of Traffic Safety, we ranked 13th out of 93 similar cities for pedestrian injury accidents, and sixth for cyclists. This is unacceptable for a community that characterizes itself as a “village.”

I urge members of the Los Altos City Council to use their discretion to keep the current posted speed limits, as they wisely did in 2007. They should instead use the traffic survey data to develop measures to slow drivers down. This would be a decision based on the data and sound physics.

Daniel Burns is a Los Altos resident who teaches Advanced Placement Physics at Los Gatos High School.

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