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Study: Rail Transit May Slow Growth in Traffic Congestion

Light Rail Progress – March 2001


it is a repeated contention of zealous opponents of rail transit that rail transit (particularly light rail), as well as mass transit in general, will have negligible impact on traffic congestion. In the ideology of these opponents is the implication that free-flowing, uncongested conditions will somehow be restored to urban areas, particularly if scarce dollars are diverted away from supposedly ineffective alternatives like light rail and funnelled into the roadway system.

Light rail transit (LRT) supporters are quick to point out that, since light rail trains bypass traffic lanes stacked up with cars, LRT is an alternative to traffic congestion. And New Urbanists argue that, by attracting clusters of development and promoting better urban design, light rail is a powerful tool in attenuating the sprawl which promotes congestion.

Nevertheless, the consensus among most transportation planners, both in this country and abroad, seems to incorporate the recognition that population and vehicular growth imply that some degree of congestion will be inevitable; in other words, traffic congestion is here to stay. Short of a catastrophic event (like World War 2), we'll never see the free-flowing traffic conditions of 5 or 6 decades ago. (Whatever roadway seems "uncongested" rapidly fills up, as both motorists and developers rush to take advantage.) Thus, the best that can be hoped for is somehow to slow the growth of congestion.

Rail Transit May Slow Congestion Growth

Rail transit in US cities is relatively minuscule, having been the target of wholesale decimation in the period 1930-1960, and having seen restoration of only a small fraction of route-mileage in transit systems in the recent period. Nevertheless, within the perspective of attentuating traffic congestion growth, is there any evidence that even this minuscule presence of rail transit has an influence?

While traffic congestion is indeed growing in all cities, an examination of data for large and very large urban areas compiled by the Texas Transportation institute (TTI) at Texas A&M University indicates that, in those urban areas with rail transit in major traffic corridors, there is a lower rate of congestion growth than in similarly sized urban areas without rail.

Large and very large US urban areas with rail transit systems serving major travel corridors show a significantly lower rate of congestion growth than cities without rail transit. in these urban areas with rail, traffic congestion appears to be increasing, but at a rate 42% lower than in similar urban areas without rail, according to a study completed in October 2000 by Mobility Planning Associates of Austin, Texas. The study results clearly have implications for cities considering the installation of new light rail systems.

Traffic congestion is increasing virtually everywhere – whether cities have only expanded their high-capacity freeway and arterial systems, or have installed or expanded rail transit also. But the TTI data indicate that, for large and very large urban areas, the rate of congestion increase is substantially lower in cities that have either light rail or "heavy" rail subway/elevated lines in major travel corridors than in those cities without such systems.

Study Criteria

MPA's study of the TTI data looked at urban areas designated as "Large" or "Very Large" by TTI (since all cities meeting the rail transit criteria fell into those categories). Small and medium-sized urban areas (as designated by TTI) were excluded, since travel conditions in such cities may not fairly be compared with those in the large and very large areas.

The study looked at the change in TTI's preferred index of traffic congestion, the Travel Rate index (TRi), for the period 1992-97. As TTI explains, the TRI is a ratio of roadway travel in peak, congested conditions to travel in uncongested, free-flowing conditions. The TRI is therefore a relatively effective and plausible measure of congestion.

TTI's own analysis summarizes changes in TRI for specific periods. The 1992-97 period (a standard study period designated as "Short-Term" by TTI) was plausible since it enabled MPA to evaluate possible impact for new rail systems coming online in the early 1990, such as those in St. Louis and Denver. Thus almost all major new rail transit system have had time to demonstrate at least several years of performance in that period. (in contrast, it should be noted, studies by rail opponents and critics, such as recent reports by Wendell Cox and Tom Rubin published by the Texas Public Policy Foundation, often look at trends for longer periods, during most of which rail service was not initiated in many cities included in the survey period.)

However, Dallas was excluded from this study, since the city's new DART light rail system, installed in mid-1996, had a full year of operation only in 1997, the last year of the 5-year period under study (with only a portion of the starter system in service) – thus it is unreasonable to expect any measurable impact on Dallas traffic. During the 1992-97 period, the Dallas area's TRI increased 41%.

Likewise, downtown circulator trolley and "people-mover" systems, such as those in Seattle and Detroit, were excluded, since they do not serve major traffic corridors and could not reasonably be expected to impact congestion. On the other hand, Ft. Worth's privately owned surface-to-subway light rail line, although very short, was included, since it does serve a major corridor on the north end of the CBD, and has been in operation since 1963.

For the 1992-97 period examined, traffic congestion, as measured by the TRi, increased 55.9% in urban areas without rail transit, but only 32.4% in urban areas with rail transit in major travel corridors. in other words, traffic congestion grew at a rate 73% higher in non-rail cities, than in cities with rail in one or more major travel corridors.

Urban areas with rail transit in major travel corridors included: Cleveland, Philadelphia, Baltimore, New York, Los Angeles, San Diego, Atlanta, San Francisco, Washington, Boston, Chicago, Pittsburgh, St. Louis, Ft. Worth, Denver, Portland, Sacramento, San Jose, Buffalo, New Orleans, Miami.

Non-rail urban areas included: indianapolis, Detroit, Columbus, Cincinnati, Houston, San Antonio, Minneapolis, Seattle, Kansas City, Oklahoma City, Norfolk, Orlando, Milwaukee, Ft. Lauderdale, San Bernardino, Phoenix, Las Vegas.


Urban Area % Change in TRI
Cleveland 125
Philadelphia 29
Baltimore 35
New York 25
Los Angeles 2
San Diego 3
Atlanta 127
San Francisco 0
Washington 14
Boston 28
Chicago 19
Pittsburgh 0
St. Louis 85
Ft. Worth 33
Denver 56
Portland 43
Sacramento 26
San Jose -3
Buffalo 33
New Orleans -5
Miami 6


Urban Area % Change in TRI
Indianapolis 144
Detroit 19
Columbus 62
Cincinnati 29
Houston 36
San Antonio 88
Minneapolis 100
Seattle 5
Kansas City 125
Oklahoma City 125
Norfolk 50
Orlando 11
Milwaukee 17
Ft. Lauderdale 50
San Bernardino 8
Phoenix 33
Las Vegas 48

Rev. 2001/03/14


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