In fact, additional freeway capacity cannot solve traffic congestion. In 1992 Anthony Downs coined the term triple convergence in his book Stuck in Traffic: Coping with Peak-Hour Traffic Congestion. Triple convergence describes how peak period traffic congestion is inevitable because drivers will compensate for capacity increases by (a) shifting routes, (b) shifting time of travel, and (c) shifting travel mode. After capacity expansion, the new equilibrium will be just as congested as the old equilibrium.

Downs writes that drivers will choose “limited-access roads that are faster than local streets if they are not congested,” but the attractiveness of such routes will cause them to become congested “to the point where they have no advantage over the alternate routes.” This means that freeways posted at 55 mph or higher will instead operate at the same average speeds as local streets during peak travel periods. We call many of these roads “Interstate Highways” but very little peak period traffic on these roads is long-distance travel. Most of the traffic is from residents making routine daily trips. Adding freeway capacity sucks more of these local travelers onto the freeways. It does not reduce congestion.

Freeway congestion is not uniform but instead is focused at and behind bottlenecks. Many of these bottlenecks recur daily, with common locations including backups behind on-ramps, ahead of on-ramps, in merge and weave areas, and in lane drops. Driving on the freeway system during peak periods typically includes experiencing multiple bottlenecks. If congestion is not too severe, traffic moves relatively smoothly in stretches – until the next bottleneck is reached.

Those who drive daily in congested conditions have very complex mental models of freeway congestion. We consider the bottleneck locations and estimate delays behind these bottlenecks based on the day of the week, the time of day, the weather, and special events. We also consider uncertainty. For example, if we get on the freeway and congestion is worse than usual, there could be a very long delay. To avoid this risk, we might instead use a non-freeway route. In addition to our mental models, many also are now using Google Maps and other GPS apps to help plan routes based on real-time traffic data.