Causes and cures of highway congestion

Abstract

P eople believe congestion occurs because demand exceeds capacity, so they support initiatives to build additional highway capacity or curtail highway travel demand. Politicians work to bring highway construction projects into their districts; environmentalists support proposals to make transit more attractive or automobile use more costly. This article argues that the facts do not support the belief that congestion occurs because demand exceeds capacity.On the contrary, the major cause of congestion is the inefficient operation of highways during periods of high demand. Analysis of data shows that congestion reduces highway efficiency by 20 to 50%; that is, vehicles take between 20 and 50% more time to traverse sections that are congested than they would if congestion were prevented. Compensating for this efficiency loss through, say, a 20% capacity expansion is financially impossible; compensating through a 20% demand curtailment is politically impossible. The best way to combat congestion is through increases in operational efficiency. To increase efficiency, however, it is neces-sary to intelligently control access to highways through ramp metering.We estimate that for Los Angeles, the annual congestion delay is 70 million vehicle-hours. If the highways were to be operated at 100% efficiency, this delay would be reduced by 50 million vehicle-hours.The article is organized as follows. We first show that vehicles travel at 60 mph when there is maximum flow on a highway section (i.e., when the section is operating most efficiently). Thus, congestion delay should be measured as the extra time vehicles spend on the highway traveling below 60 mph: a vehicle taking 20 minutes to travel 10 miles at 30 mph suffers a congestion delay of 10 minutes.The efficiency of any highway section may then be defined aswhere VMT is the total number of vehicle-miles traveled and VHT is the total number of vehicle-hours traveled over the 26 IEEE Control Systems MagazineDecember 2001 Varaiya

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