Comments on: Freeing Up ‘Freeways’ http://www.howwedrive.com/2009/01/08/freeing-up-freeways/ Just another WordPress weblog Thu, 17 May 2012 05:51:06 +0000 http://wordpress.org/?v=2.5.1 By: aaron http://www.howwedrive.com/2009/01/08/freeing-up-freeways/#comment-5514 aaron Sun, 11 Jan 2009 16:14:57 +0000 http://www.howwedrive.com/?p=505#comment-5514 A simple way to explain this is that people all use some form of the equation for stopping distance we learned in drivers ed to allow safe following distance to react to traffic changes. distance = velocity * reaction time + braking coefficient * velocity^2 + some constant Reaction distance increases linearly with speed, slowing distance increases quadratically. Once the speed is reached where reaction distance is equal to stopping distance, more speed requires much more stopping distance. So at high speeds, more space is needed to follow safely. At 45mph, much less space is needed than at 70, so more cars fit onto the roads. Also, once capacity for a given speed has been reached, when new cars enter the system, traffic must slow down to make space. This is true whether traffic is flowing above or below the ~45mph threshold. So preventing new cars from entering the system allows traffic to flow at a more efficient speed. Interestingly, the 45mph speed is very near the optimal speed for fuel efficiency. 55mph gets optimal fuel efficiency, but there is little difference between 55 and 45mph efficiency. Below 55mph, faster is more fuel efficient. This is probably a big part of why high gas prices cause fuel efficiency to go down, as has been happening for several years. Despite less driving and more efficient vehicles being bought. People mistakenly think slow is more fuel efficient, both in acceleration and speed. Accelerating quickly, but smoothly, is more efficient than accelerating slowly. And faster is more efficient except for freeway speeds above 55mph. When people bring this slow is efficient mentality to the surface streets, congestion at bottlenecks compounds this problem. A simple way to explain this is that people all use some form of the equation for stopping distance we learned in drivers ed to allow safe following distance to react to traffic changes.

distance = velocity * reaction time + braking coefficient * velocity^2 + some constant

Reaction distance increases linearly with speed, slowing distance increases quadratically. Once the speed is reached where reaction distance is equal to stopping distance, more speed requires much more stopping distance. So at high speeds, more space is needed to follow safely. At 45mph, much less space is needed than at 70, so more cars fit onto the roads.

Also, once capacity for a given speed has been reached, when new cars enter the system, traffic must slow down to make space. This is true whether traffic is flowing above or below the ~45mph threshold. So preventing new cars from entering the system allows traffic to flow at a more efficient speed.

Interestingly, the 45mph speed is very near the optimal speed for fuel efficiency. 55mph gets optimal fuel efficiency, but there is little difference between 55 and 45mph efficiency.

Below 55mph, faster is more fuel efficient. This is probably a big part of why high gas prices cause fuel efficiency to go down, as has been happening for several years. Despite less driving and more efficient vehicles being bought.

People mistakenly think slow is more fuel efficient, both in acceleration and speed. Accelerating quickly, but smoothly, is more efficient than accelerating slowly. And faster is more efficient except for freeway speeds above 55mph.

When people bring this slow is efficient mentality to the surface streets, congestion at bottlenecks compounds this problem.

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