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Denver Data Show Light Rail's Real Impact on Mobility Congestion

Light Rail Progress – November 2002

Denver's Regional Transportation District (RTD) has provided Light Rail Progress with extremely useful information relating to the impact of Denver's new Southwest light rail transit (LRT) line on traffic congestion in one of the major corridors it serves. This information yields insight into how LRT can have a significant impact on traffic congestion, and can contribute to improving mobility in an affected transportation corridor.

Santa Fe Drive Corridor

The Denver RTD data, from the fall of 2000, focus on traffic flow on a section of Denver's Santa Fe Drive (north of Mississippi), a signalized 6-lane major arterial parallelled by the Southwest LRT line to Littleton on a separate, exclusive railroad right-of-way. The segment of roadway covered by the corridor study is 3 lanes wide, eventually narrowing to 2 lanes.

The Southwest LRT line runs in a separate, exclusive alignment along an existing railroad right-of-way, through industrial areas, and past residential developments which range from lower-density urban housing to suburban housing. in the peak hour during the RTD study period, eight trains per hour at 6 two-car trains and 2 three-car trains were running along this alignment – a total of 18 cars per hour.

Bus ridership in the Sante Fe Drive corridor was roughly 2,000 passengers per day before the Southwest LRT line opened. Since the Southwest Line opened in July 2000, LRT replaced bus service in the corridor, with bus feeder service interfacing with the LRT at stations. in October 2000, covered by the period of the RTD study, LRT ridership averaged over 13,000 riders per day.

Nearly 1/3 of Passenger Traffic by LRT

According to data from the Colorado Department of Transportation, peakhour, peak-direction roadway traffic volume on the section of Santa Fe studied is approximately 4,500 motor vehicles. RTD does not now run buses on this segment. However, approximately 7% of this traffic consists of freight vehicles. Thus the automobile count is about 4,180 vehicles. With average auto occupancy of 1.2 persons per car, the number of persons travelling by automobile is approximately 5,020.

RTD tabulates the number of peakhour, peak-direction LRT passengers at 2,000 to 2,500. Light Rail Progress calculates, therefore, that total peakhour, peak-direction person-movement in the corridor ranges between 7,020 and 7,520. Of this, the percentage (modal split) travelling by LRT ranges between 28.5% and 33.2% – a significant proportion of total passenger traffic.

Average Proportion of LRT vs. Automobile Passenger Peak Traffic in Denver's Santa Fe Drive Corridor

Put another way: if these riders chose to make their trip by automobile instead, they would further congest these 3 lanes of Santa Fe Drive with between 1,670 and 2,080 additional automobiles during this peak hour. Thus, LRT can be said to provide a significant amount of "congestion relief" in this case.

Goal: Relieve Mobility Congestion

LRT critics and opponents consistently try to pose "reduction" of roadway traffic as a basic measuring stick for the "success" of LRT – a measure it will inevitably fail to meet. in reality, by raising (unachievable) expectations of significant roadway congestion reduction from LRT and other major transit projects, transit and LRT opponents exploit a common fallacy and misconception: That any single transportation facility, roadway or transit, can ever truly "reduce" congestion. it is almost universally recognized, even among highway planners, and throughout the transportation planning profession, that roadway traffic congestion is a fundamental fact of life – basically, it continues to grow with population expansion and the proliferation of motor vehicles. Acceptance of some degree of congestion is actually incorporated into the basic design of urban roadways.

For these reasons, bona fide congestion relief provided by LRT and other major transit services cannot be expected to take the form of significant reductions in road traffic. instead, relief is far more likely to take the form illustrated in Denver: diversion of significant traffic growth into high-quality transit service in specific corridors. (We have no specific information one way or the other of any fluctuation in motor vehicle volumes on Santa Fe Drive.)

it is either an error or a deception to try to assess congestion relief by the measure of whether or not existing congestion simply evaporates. Congestion never just "evaporates". The traffic lanes on Santa Fe Drive are probably as crowded as ever (particularly because of ongoing population and traffic growth throughout the metro area). What LRT does is to open up, in effect, a new "traffic artery" along which people can move past the existing congestion. Moreover, unlike the capacity-increasing effects of a freeway, the result with LRT is that all those cars are off the road, out of the traffic stream, and out of the competition for scarce parking spaces. Perhaps the realistic goal of major transit improvements like LRT, therefore, is to relieve mobility congestion, and not necessarily traffic congestion.

Lessons for Other Cities

in sum, these data from Denver suggest that LRT at peak hour in the peak direction in the target corridor is carrying between 28% and 33% of the total passenger traffic flow. in other words, without the LRT line in service, approximately 30% of corridor passenger traffic would be added to the roadway congestion. We believe this demonstrates that LRT can have a significant impact on corridor traffic congestion.

These results have significance for other communities evaluating LRT and other mobility improvements. The public might well consider whether they would rather have an additional 30% (or other percentage) of motorists on crowded streets, contesting for scarce space. Expanding the roadway arterial to accommodate this extra traffic in Denver's case would mean adding at least 4 to 6 more lanes to Santa Fe Drive – an extremely expensive proposition, with very costly inner-city right-of-way acquisition as well as construction. And what about the additional parking spaces for thousands of additional cars?

Bottom line: As the Denver case demonstrates, LRT can have a very real impact on congestion – a lesson which other areas might well take to heart.

Light Rail Progress wishes to acknowledge the use of photos from Kavanagh Transit Photos (LRT on Littleton line) and Jon Bell (LRT at Mineral Ave. Station).

Rev. 2002/11/24

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