Transportation Air Quality Selected Facts and Figures
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Is Traffic Congestion in Our Cities Getting Better or Worse?
Congestion occurs when the free flow of traffic on a roadway is impeded due to excess vehicle demand, construction, maintenance, traffic incidents, weather, or other road conditions and events.
Before the 2008 economic recession, congestion was increasing at between 2 and 4 percent every year–which meant that extra travel time for the average commuter increased slightly less than 1 hour every year. The economic recession set back that trend a few years, but the trend in the last few years indicates congestion is rising again.
This map shows the percent change from 2011 to 2014 in the amount of time per trip it took to travel in the peak period for selected areas.
Changes in Traffic Congestion (2011–2014)
Source: Texas A&M Transportation Institute, 2015 Urban Mobility Report, August 2015, Table 1, http://d2dtl5nnlpfr0r.cloudfront.net/tti.tamu.edu/documents/mobility-scorecard-2015.pdf
What Are the Major Sources of Congestion?
FHWA studies show that congestion is the result of seven root causes, often interacting with one another.
- Physical Bottlenecks (“Capacity”)–capacity is the maximum amount of traffic capable of being handled by a given highway section. When demand is greater than capacity, physical limitations can cause congestion.
- Traffic Incidents–events that disrupt the normal flow of traffic such as vehicular crashes, breakdowns, and debris in travel lanes.
- Work Zones–construction activities on the roadway that result in physical changes to the highway environment. These changes may include a reduction in the number or width of travel lanes, lane “shifts,” lane diversions, reduction or elimination of shoulders, and even temporary roadway closures.
- Weather–environmental conditions can lead to changes in driver behavior that affect traffic flow.
- Traffic Control Devices–Intermittent disruption of traffic flow by control devices such as railroad grade crossings and poorly timed signals.
- Special Events–demand fluctuations whereby traffic flow in the vicinity of an event (sports, concerts) will be radically different from “typical” patterns. Special events occasionally cause “surges” in traffic demand that overwhelm the system.
- Fluctuations in Normal Traffic–Day-to-day variability in demand leads to some days with higher traffic volumes than others.
Source: Federal Highway Administration, Traffic Congestion and Reliability: Linking Solutions to Problems Report, July 2004, http://ops.fhwa.dot.gov/congestion_report_04/index.htm#toc