Traffic congestion can cause a multitude of environmental and infrastructure problems.
Traffic congestion grew for 85 of the largest American cities nearly every year between 1982 and 2003. The most comprehensive traffic research of US roadways is performed by the Texas Transportation Institute, (TTI), a Texas state agency affiliated with Texas A&M University. The TTI estimates that commuters in urban locations spend over 40 hours sitting idle in rush hour each year, costing more than $78 billion.
The U.S. Department of Transportation Federal Highway Administration (FHWA) reports that 40 percent of traffic congestion is directly caused by bottlenecks. The second most common reason for congestion is traffic incidents, such as traffic crashes and breakdowns. Traffic incidents account for one quarter of all congestion. On an average three lane highway, a traffic incident that blocks one lane, or one third of the highway, reduces the capacity of the roadway by half.
The FHWA relies on the departments of transportations within each state to monitor traffic on all roads within their state. The FHWA recommends that states study traffic patterns of heavy volume sections of roadway every three years, assuming that data is collected for a portion of each year; rudimentary traffic studies are suggested in six year cycles. Traffic data collected by states are used by the FHWA Highway Performance Monitoring System to determine federal aid to state transportation agencies, record statistics on traffic safety and evaluate Clean Air Act requirements.
Permanent and portable Automated Traffic Recorders (ATRs) are installed to calculate traffic data on roadways in both rural and urban areas. Major highways often maintain ATRs every half mile. Permanent ATRs are installed under roadways, collecting data for 24 hours, each day throughout the year. The traffic data is transmitted to the assigned transportation agency each day. Portable ATRs are placed in roadways and used periodically for traffic monitoring of secondary roads. Traffic counting by the portable ATR is normally conducted over a 48-hour period.
In-pavement congestion sensors are placed in the pavement to measure congestion speed and overall volume. Large cities are able to use in-pavement sensors to monitor traffic volume at different times and locations and times of the day. This data is useful in setting traffic light timers and road design at times of reconstruction. In many areas, in-pavement traffic sensors are installed at intersections to control the traffic lights. Rather than traffic lights working strictly by timer, which may cause drivers to sit longer while there is no cross-traffic, sensors will trigger the light to allow traffic through on whichever side has vehicles waiting.
Closed circuit television (CCTV) cameras are used on highways to monitor both traffic congestion and traffic incidents. State transportation workers view the activity on roadways by monitor from CCTV cameras to divert traffic from congested or blocked lanes. Once an incident occurs, or volume causes movement to slow, operators display messages on computerized signs over roadways leading to congestion.
In the 1990s, the Texas Department of Transportation (TXDOT) tested computerized traffic signs, called Dynamic Message Signs (DMS) by posting rush hour traffic estimates. After the DMA system was introduced, TXDOT conducted a motorist survey. Of those polled, 93 percent wanted to see traffic incidents reported on the DMS and 82 percent found traffic estimates useful. The DMS traffic information convinced 85 percent of motorists to change their route and two thirds reported that the change reduced their commute.
Major cities have combated traffic by offering more reliable and expanded public transportation. Many cities also reduce traffic by redesigning their existing highways to create an efficient traffic pattern and by timing traffic lights to meet the needs of specific intersections, as well as implementing procedures facilitating the prompt removal of lane obstructions.
The Chicago Department of Transportation (CDOT) implemented the Traffic Calming Program to improve traffic safety in the city's residential neighborhoods. The CDOT built structures along roadways that are easily visible to drivers and force them to slow or merge with traffic. The traffic calming program is designed to reduce jams by guiding the flow of traffic. Slowing traffic in residential neighborhoods is an effective tool in reducing incidents.
Los Angeles traffic congestion has been reduced by the use of sensors that control freeway traffic entering and exiting ramps. The California Department of Transportation (Caltrans) began the System Wide Adaptive Ramp Metering Project (SWARM). SWARM uses monitoring systems throughout the Los Angeles area to calculate the most likely times for bottlenecks and to utilize the ramps funneling traffic in those areas. Traffic lights at freeway ramps are controlled by using calculations to set the light to change at set intervals.