Traffic Jams

Last modified: 2010-06-25 05:26:32 UTC

© 2009~2010 Charles L. Chandler

 

Traffic on the highway proceeds at a normal rate, until the number of cars exceeds a certain threshold. Then the speed drops by 50%. At the slower speed, fewer cars get to their destinations in the same period of time. So even more cars get packed onto the highway, which further decreases the speed. And this, of course, is what causes traffic jams.

The cost of traffic jams is measured in lost time for commuters, increased pollution from the vehicles operating inefficiently in stop-and-go traffic, and in a higher incidence of accidents. The quick solution is to build more highways, bridges, and tunnels. But such projects are extremely expensive, and during construction (which can take years), the problem gets worse before it gets better.

To get the most out of the highways in densely-populated areas, we have to figure out a way of keeping the volume of traffic below the critical threshold for a slowdown. This, of course, means developing a way of controlling the number of cars getting onto the highway.

The entrance ramps to freeways will have to be fitted with gates that will open to let cars through until the "car density" on the highway gets near the critical threshold. Then the gates will close, and will only open as the "car density" relaxes. As a result, commuters will have to wait for 5 or 10 minutes until allowed onto the highway, but then they'll be able to travel at full speed to their exit, instead of creeping along at 5~10 mph. And they will quickly realize that they're getting to their destinations a lot faster this way.

To make things fair for everybody, all of the entrance ramps to a freeway will have to be centrally controlled, and the number of cars entering the freeway at each ramp will have to be balanced. If we only let cars onto the highway until the threshold was reached, and then closed all of the entrance ramps further down the road until all of that traffic had moved through, then everybody would create traffic jams on the secondary roads attempting to get on the highway before the closures, and we wouldn't be any better off.

It might also make sense to throttle the access to the highway depending on the congestion on the secondary roads.

While it wouldn't exactly be free to set up gates to throttle the access to highways, and to set up the central computer system that will control them, it sure would be a lot cheaper than widening the highways to relieve the congestion as the population in our cities continues to grow.

If this strategy proved effective, what we learn might be applicable to other situations in which ad hoc traffic throttling must be used. For example, evacuations that have been ordered because of impending hurricanes are always problematic, since it's not just rush-hour commuters involved — it's everybody. This alters the nature of decision-making for emergency managers, since they have to order evacuations far enough in advance to get people out, assuming that the cars will be traveling at 5 mph. If they could assume that the average speed is going to be 55 mph, they would have more time, and could make more informed decisions.