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Photo of the Dallas North Central Expressway experiencing a bottleneck

Dallas North Central Expressway.

Table 1: Cost evaluation results. El Paso: I-10 East at US 54; auxiliary lanes; cost - $530,000; benefit/year - $1,300,000. Dallas: I-35 E., I-635 to Loop 12; Shoulder conversions; cost - $1,900,000; benefit/year - $11,000,000. Dallas: I-35 E. North, I-30 to Tollway; Auxiliary lanes; $130,000; benefit/year - $600,000. Dallas: I-30 East, I-35 E. to I-45; ramp reversal; cost - $660,000; benefit/year - $740,000. The three Dallas locations & #039; benefits per year are due to reduction in delay only. Crash data not yet available.

Cost Evaluation Results.

Figure 1: illustrates restriping improvement implemented to address a bottleneck problem

Lane Layout.

For more information:

Carol Walters
817-462-0523
c-walters2@tamu.edu

Do bottleneck improvements really reduce congestion?

What is a "bottleneck"? Congestion problems in this category can be visualized as the neck in an hourglass. Not enough capacity for just a short distance, but a constraint that stacks up traffic.

What is all the fuss about? These are projects without a dedicated funding source. Not large enough to be studied as major investments, but too large or complex to be fixed in a few hours by a maintenance crew.

To meet a recognized need of the profession to better understand the impacts of projects designed to reduce or eliminate bottlenecks, the Texas Transportation Institute (TTI) on occasion collects before and after data to determine whether an improvement in travel time or capacity has been realized. Over the course of many years, a database has been developed showing results of selected Texas Department of Transportation (TxDOT) initiatives to remove bottlenecks and improve the operations of urban freeways. Many lessons have been learned from examining this database.

An example is a recent effort in the Dallas district to improve merging/weaving at the entrance to southbound US 75 (North Central Expressway) from the recently constructed President George Bush Turnpike. The problem was at its worst during the morning peak period, as is shown in the photo to the left obtained by the North Central Texas Council of Governments, which sponsored a Low-Level Aerial Photography Assessment of the entire Dallas/Fort Worth urbanized area in 1999. Not shown in the photo is the cause of the bottleneck, which was a forced merge of the ramp traffic into the southbound mainlanes of US 75.

Figure 1 illustrates the solution developed by TxDOT in conjunction with the City of Richardson. The restriping improvement was implemented recently, and TTI was asked to evaluate the benefits. Before and after data established that each vehicle using the ramp connection averaged one minute in travel time savings, with a peak savings of over three minutes. At the same time, mainlane traffic maintained or enjoyed a slight increase in speed; volumes increased on both facilities. An annual benefit of over $500,000 in delay savings was calculated based on morning peak period traffic alone, compared to an implementation cost of less than $20,000.

Why are more of these types of projects not being implemented wherever bottlenecks exist? Although the dollars are small, there are no dedicated moneys for them; implementation comes from the maintenance budget, and potholes and damaged guard rails must take precedence in a budget that is constrained. TTI's research effort has been directed toward review of benefits that result from the selected projects that have been evaluated. Eventually, this may provide planners adequate data to consider a wider program. A sampling of other evaluation results is included in Table 1.

In Table 1, the ramp reversal designed by TxDOT converted a little used exit ramp to an entrance ramp from the eastbound collector roadway paralleling the mainlanes in the "Canyon" near downtown Dallas. This allowed bypassing a constrained weave section that was backing up I-35E southbound. Additionally, the weave occurs on a fairly tight curvature, a contributor to rollover problems for trucks during off peak time periods. Crash records will be analyzed in more detail when available.

In general, benefit-to-cost ratios for these projects are typically high, averaging around 20:1, with a range from 5:1 to 200:1, for a ten-year life. Costs are low, sometimes only for restriping, and there are thousands of freeway users who benefit from quicker and more reliable travel time. Further, in some cases, reductions in crash rates have been noted over time, adding to the benefit.

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