United States Department of Transportation
i
Heavy vehicle performance during recovery from forced-flow urban freeway conditions due to incidents, work zones and recurring congestion.
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2013-12-01
[PDF-3.01 MB]
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Abstract:Information contained in the Highway Capacity Manual on the influence heavy vehicles have on freeway traffic
operations has been based on few field data collection efforts and relied mostly on traffic simulation efforts. In the
2010 Manual heavy vehicle impact is evaluated based on “passenger car equivalent” values for buses/trucks and
recreational vehicles. These values were calibrated for relatively uncongested freeway conditions (levels of service
A through C) since inadequate field data on heavy vehicle behavior under congested conditions were available.
Field-collected headway information was based on the average headway for vehicles in a particular class,
regardless of the type of vehicle they were following.
The goal of the present effort was to collect and analyze freeway field data on headways with an emphasis on
heavy vehicle behavior under lower speeds typically associated with a level of service E (capacity) or F (forcedflow
conditions). Contrary to previous efforts incorporated in the Highway Capacity Manual methodology, that
disregarded the effect a leading vehicle has on headways, headway information was collected for ten
leading/following vehicle pair types containing at least one passenger car (for example buses followed by
passenger cars). Headway statistics were analyzed for nine speed ranges (up to 20 mph; 20-25 mph; 25-30 mph;
30-35 mph; 35-40 mph; 40-45 mph; 45-50 mph; 50-55 mph; and 55+ mph) and ten vehicle pair types.
Passenger car equivalent values were derived for each speed range based on the average headway for a specific
vehicle pair type divided by the average headway between passenger cars. A total of 3,981,810 individual vehicle
records were used to construct the 2,645,210 vehicle pair records for which headway statistics were compiled. All
analyzed information originated from the Milwaukee County, Wisconsin urban freeway system. Leading and
following vehicle class and speed were found to significantly influence headways and passenger car equivalent
values.
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