How Traffic Works

Travelers in Maryland are warned of changes in traffic patterns as they travel on a seven-mile, two-hour traffic jam caused by construction of a new bridge.

Photo Jim Watson/AFP/Getty Images

Traffic Solutions

Many cities, like Los Angeles, have sophisticated traffic communications systems that alert drivers to changing conditions on the road, giving them time to decide what to do. Several cities have invested millions of dollars so that road crews can rapidly travel to trouble spots.

There are a few ways cities can address highway congestion:

  • Ramp metering - cars are only allowed to enter the highway at timed intervals. This is done by placing a light similar to a traffic signal at the end of the ramp. Cities using ramp metering report a travel-delay reduction of 29.4 million hours annually with a significant decrease in traffic accidents -- yet capacity on the highway actually increased when ramp metering was introduced [source: TTI]. In Minnesota, the DOT instituted a strict ramp-metering program that included 430 ramp meters. As an experiment, the DOT shut off the ramp meters for seven weeks in 2000. During that time, traffic accidents increased by 26 percent. After the experiment, the DOT reinstituted the ramp meters and saw highway capacity increase by 14 percent [source: TTI]. Though ramp metering can increase highway speeds while decreasing accidents, it usually takes a long time to implement and requires thorough investigation to make sure surface street traffic isn't affected as cars back up to enter the highway [source: AGC of America].
  • High-occupancy vehicle lanes (HOV) - many cities have included these lanes on highways. HOV lanes are reserved for cars with a certain number of passengers (usually two or three people per car). Drivers have an incentive to carpool, reducing the total number of cars on the highway. Some HOV lanes have exclusive off-ramps, reducing the need to merge with other traffic.
  • Adding lanes - a common approach to congestion problems is to add lanes to the highway, either by widening the road, decreasing the width of existing lanes or converting a shoulder or other space into a lane. These sort of adjustments are expensive, time consuming and controversial. Several studies suggest that increasing the width of a road only increases the volume of cars without addressing congestion. Other studies say that in many circumstances, widening the road can greatly reduce congestion. According to TTI's extensive research, adding lanes and widening roads only works if capacity stays ahead of population growth [source: TTI].

In the next section, we'll look at congestion on city streets. ­­