A fascinating article in Slate provides evidence suggesting that slower highway speed limits may actually get you to your destination more quickly! This apparent paradox has some seemingly good evidence behind it. For example, some recent trials on a congested highway in Colorado had highway patrol vehicles  “riding in tandem with lights ablaze” travelling at 55 mph. This “pacing” (rolling speed harmonization) is based on the idea “that by encouraging speed compliance and reducing speed differential between vehicles, volume throughput can be maximized without a physical increase in roadway dimensions.” The notion builds on a traffic engineering principle that large speed differentials are dangerous. “Studies show that, regardless of the average speed on the highway, the more a vehicle deviates from the average speed, the greater its chances of becoming involved in a crash.”

So why not just have everyone travelling at the same fast speed and you have the best of both worlds?

The answer come from another ‘law of traffic’ – that it takes exponentially longer to get out of a traffic jam than to get into one. Rather than having drivers go full-tilt into a jam at a tunnel entrance, for example, drivers would approach more slowly. Thus, even though their speed is reduced, the system is now processing vehicles faster. The writer cites the ‘rice-and-funnel’ effect popularized by a former U.S. transportation commissioner: The slower your pour the rice, the faster it gets through the bottleneck.

Apparently several European studies show that technologies that generate varying speeds (VSL) depending on traffic conditions, do under the right conditions, help reduce crashes and improve highway throughput (even when mean speeds are lowered). Generally, traffic engineers assert that a highway can move more vehicles per hour at 55 mph than 85 mph. The problem is getting drivers to change their speed. A VSL system needs robust enforcement to maximize speed compliance. In Europe, automated speed cameras and similar technologies typically do the enforcement. However, as one expert noted, “that’s not going to sell very readily over here.” Speed cameras are a tougher sell in the United States because Americans relish their freedoms—even if it’s the freedom to drive into a traffic jam.

For more on this intriguing idea check –

http://www.slate.com/articles/life/transport/2011/10/rolling_speed_harmonization_how_colorado_fights_congestion_on_i_.html