The Environmental Consequences of Traffic Congestion

Jennifer Hogan

In the fifteen-year period from 1982 to 1997, vehicle miles traveled (VMT) each day in the Providence metropolitan area increased from three million to seven million, a 133 percent increase, while population in this area increased only nine percent.  The rest of the VMT increase resulted from more and longer trips with lower vehicle occupancy.  The consequence has been an increase in annual hours of delays in traffic from 3 to 17 hours per capita and a five-fold increase in annual congestion costs, from $50 to $250 per person.  Congestion also increased fuel consumption (an additional 19 million gallons of gasoline consumed in 1997 as a result of congestion compared to 1982) and air emissions.

The two most common responses to traffic congestion are to provide more roads and encourage mass transit.  I analyzed a third alternative to reduce congestion: control of traffic flows by the placement and synchronization of traffic controls. Creative use of existing roadways can increase capacity and discourage congestion.

 I was able to calculate air pollutant emissions and, in some cases, fuel consumption to demonstrate the impacts of increased congestion in the Providence metropolitan area and to evaluate traffic control methods.  Based on EPA’s MOBILE5a model's estimates for the year 1999, emissions in the downtown area due to congestion were about one-third of the emissions from moving traffic.  I calculated the amount of carbon being emitted into the atmosphere from "excess fuel consumed" for the metropolitan area; it had increased by about five times between 1982 and 1997.

In Providence, stop sign maintenance and installation is not well recorded and a stop sign’s position is only reevaluated on routine maintenance calls.  The city of Providence does not conduct any large-scale comprehensive assessment of traffic flow.  I used a traffic computer model called NETSIM, to evaluate the emissions and fuel consumption impacts of stop signs.  Using traffic data from the Fox Point roadways as an example, I found that fuel consumption and pollutant emissions increased when a stop sign caused delay and/or stopping.  When adding a stop sign, I found that local roads experienced an increase of approximately 0.3 to 0.4 kg of CO emissions, 0.1 kg VOC emissions, but not a significant change in NOx emissions. 

Traffic calming is an attractive alternative to stop signs because it can reduce emissions and fuel consumption, and also slow traffic and therefore decrease risk.  The use of traffic circles (a common traffic calming device) has resulted in a reduction of emissions and fuel consumption.  When traffic circles were compared to 2-way stop intersections, the traffic circles operated better than the intersections overall; cars had fewer accidents, emitted less pollutant, and maintained higher fuel efficiencies. 

Light synchronization minimizes delay and excess fuel consumption.  The state, in 1994, calculated emissions comparing arterial progression with no progression and found significant emissions reductions in the short and long term with synchronization.  For light synchronization on Route 44, the state predicted emissions reductions for VOC (approximately 40 kg per day), NOx (approximately between 0.5 and 0.35 kg per day) and CO (approximately 400 kg per day) in 1996 and 1999. 

Overall, I found that emissions decreased with traffic signal progression, at higher traveling speeds, and with lower VMTs.  Arterial progression also proved to have emissions benefits.  Stop signs had a negative impact, creating more emissions and increasing fuel consumption.  Traffic calming can be a useful alternative to stop signs.  Techniques that do not require cars to stop or decelerate unnecessarily, such as speed bumps, can result in the reduction of emissions.

            Traffic modeling programs such as NETSIM can be used in the state and municipal government to make more informed decisions about traffic devices.  The city should consider implementing light progression for part of the day throughout the city.  Interagency cooperation between Traffic Engineering, Providence Planning and Development, and the Rhode Island Department of Transportation could help Providence evaluate transportation more comprehensively.