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Methodology

For this report STPP performed an analysis of each project to examine the effects of construction on individual commuters. This analysis required several pieces of data: existing travel speeds; projected future, post-construction travel speeds; traffic volumes; anticipated delays due to construction; construction segment length; construction period; and, the expected design life of each newly reconstructed roadway. Most of this data was available in traffic studies provided to us by the state departments of transportation. Where travel speed data was not available, we used highway modeling tools to estimate speeds based on average daily traffic (ADT). In all cases, we kept our estimates conservative.

To calculate the effects of construction projects on individual commuters STPP compared the time it took to travel the segment before construction crews broke ground, to the time highway engineers predict that it will take once the segment is completed. In most cases, individual commuters realized several minutes of improvement initially, but this improvement was greatly diminished throughout the years as traffic volumes increased. Summing this improvement over the design life of the newly reconstructed roadway gives us the total projected benefit accruing to each individual commuter.

Delay times were provided to us by state department of transportation officials and added to baseline travel times to arrive at the total travel time for individual commuters. We discovered that in most cases state DOTs have not done any detailed analysis of the construction delay time, an indication of the low priority they give this delay.

We used the most conservative estimates we were given, but delay times may be significantly higher due to unforeseen construction problems, poor weather or other factors. Summing these hours of delay over the construction period (number of years expected to complete a given project) gives us the amount of time each individual lost sitting in traffic because of the project.

The net time savings (or time lost), then, is the difference between the total time savings over the design life of the new roadway and the total time lost over the construction period. To determine the break-even year, we calculated the number of years it would take a typical commuter to accumulate a time savings which was equal to the time lost in construction delays.

Estimates of induced travel were made using DeCorla-Souza and Cohen’s speed-flow model of induced and diverted traffic.³⁰