Comment 91361

By kevlahan (registered) | Posted August 25, 2013 at 14:58:57

It is often claimed that lower average speeds (e.g. due to lowering the speed limit from 50 km/h to 30 km/h or introducing traffic calming) makes streets safer, but at the price of higher pollution.

This seems intuitive since it is based on the idea that engines are more efficient at higher speeds and that at higher speeds a car passes more quickly through the neighbourhood.

However, research suggests that lower average speeds combined with appropriate traffic calming to reduce agressive driving (fast braking and acceleration) actually leads to lower emissions. This is because the greatest factor determining emissions is not vehicle speed, but the amount of rapid acceleration. In addition, traffic calming can reduce pollution in the longer run by reducing the number of vehicle trips on the road as it becomes a less desirable "shortcut" and walking and cycling become more attractive.

Not all traffic calming is equal, however, from the emissions point of view.

Speed humps can increase emissions (due to slowing and accelerating), so it is better to implement traffic calming that encourages a uniformly lower speed (like narrower lanes or fewer lanes). Stop signs are also bad, as they mean more decelerating and accelerating. Traffic circles are better than traffic lights and stop signs for emissions (and well-designed traffic circles can effectively calm traffic by smoothly slowing it), but can pose problems for crossing pedestrians if not properly designed (e.g. if they don't slow traffic to pedestrian safe speed).

The current fast, one-way under capacity multi-lane streets like Cannon are the worst of all worlds. They encourage dangerously high speeds (for pedestrians and cyclists), and the high speed clumps of traffic encourage agressive acceleration and deceleration as motorists rapidly accelerate to try to catch the green wave and keep up with the cohort of fast-moving traffic.


Comment edited by kevlahan on 2013-08-25 15:00:28

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