Driving Shortcuts May Not Get You There Faster

With today's drivers having all the latest driving conditions at their fingertips, getting from one place to another via the shortest route should speed up drive times. However, according to new research those shorter routes may just slow everyone down.

In today's world, drivers have at their disposal various kinds of real-time information on traffic conditions, from road sensors to satellite-navigation systems that analyze how fast anonymous mobile phones pass from one phone mast to another so that motorists can use the shortest routes to their destination. Now, new research done by Hyejin Youn and Hawoong Jeong, of the Korea Advanced Institute of Science and Technology, and Michael Gastner, of the Santa Fe Institute shows that those driving shortcuts won't necessarily get people where they are going any faster and instead could slow down the flow of traffic. Their study will be published in an upcoming issue of Physical Review Letters and what they've concluded is that quicker routes could create delays and make hold-ups along an entire road network. The best way to describe their findings is to use an their example:

trying to reach a destination either by using a short but narrow bridge or a longer but wide motorway. In their hypothetical case, the combined travel time of all the drivers is minimised if half use the bridge and half the motorway. But that is not what happens. Some drivers will switch to the bridge to shorten their commute, but as the traffic builds up there the motorway starts to look like a better bet, so some switch back. Eventually the traffic flow on the two routes settles into what game theory calls a Nash equilibrium, named after John Nash, the mathematician who described it. This is the point where no individual driver could arrive any faster by switching routes.

In another test of their theory, they chose to travel from Harvard Square in Boston to Boston Commons. They found 246 different links along the road network and then added in other variables such as traffic flows and different times of the day and came up with what they call the "price of anarchy" (POA). This is the ratio of the total cost of the Nash equilibrium to the total cost of an optimal traffic flow directed by an omniscient traffic controller. They discovered that motorists driving in Boston during a time of high traffic faced a POA in which their trip was 30% longer than those who were placed into optimal traffic flow. Those motorists in London (between Borough and Farringdon Underground stations) ended-up with a POA of up to 24% and in New York (from Washington Market Park to Queens Midtown Tunnel), the POA was near 28%. By changing the road network, delays could be reduced, especially by closing some roads. This is known as Braess’s paradox, after another mathematician, Dietrich Braess, who found that adding extra capacity to a network can sometimes reduce its overall efficiency. Going back to Boston, the researchers wanted to see if the paradox could be created via closing some of the 246 links. 240 of their cases proved that road closures created driving problems. However, out of the six remaining streets, they were able to reduce the POA of the new Nash equilibrium regardless of which street they closed. Similar results were gained in London and New York. More research is needed to better understand these effects. In the meantime,

planners should note that there is now evidence that even a well intentioned new road may make traffic jams worse.