Development of a Mobility-Based Service Measure for Freeway Facilities

Abstract

The Highway Capacity Manual (HCM) currently uses a bipolar approach to defining level of service (LOS) for freeway facilities: either (1) weighted density or (2) assigning LOS F if one or more segments experience LOS F. The major shortcoming of this approach is that density is a poor indicator of travelers’ experiences under congested conditions; speeds or travel times are more relevant to travelers and are consistent with how agencies measure and report congestion using field data. Likewise, saying a facility operates at LOS F if one only segment experiences LOS F is misleading. Further, five of the six LOS ranges exist where speeds are relatively high (above approximately 50 mph) and only one LOS range is used to define the congested regime, which is of the highest interest in large urbanized areas. Especially with regard to operations improvements, many congestion management techniques will improve congestion (e.g., delay) but the facility will still be classified as LOS F under the current definition.This paper deals with this issue by defining a travel time-based service measure for freeway facilities. Two HCM applications are explored: (1) traditional(static) freeway analysis and (2) the new travel time reliability (TTR) analysis procedure. Several performance measures are explored for the service measures by analyzing field data from seven U.S. urban areas. Although the research is not complete as of this writing, the mean travel time index is probably the best measure for the static procedure and either the 80th or 95th percentile travel time index should be used with the reliability procedure. Analysis of field data revealed strong correlation between the mean and upper percentiles of the travel time distribution, so these measures are internally consistent.A shift away from the current density-based LOS structure is recommended. The new structure uses ranges of the selected travel time measures that indicate different levels of the user experience. This approach is similar to what is done in the HCM for urban streets. Reconciling LOS concepts between freeways and urban streets will make the HCM more usable for the emerging field of performance management.It is recommended that travel time reliability be the service measure for freeway facilities which are part of an extensive freeway network in large urbanized areas, those over 500,000 population. An extensive freeway network is defined as having at least three freeway facilities. Analysts should use their judgment in cases that do not meet these Density would continue to be used on all other freeway facilities as well as for other aspects of freeways covered by the HCM (e.g., freeway segments).Recognizing that agencies may want to define their own service standards, a procedure for setting locally-defined standards is developed, based on the steps used in this research.It is expected that the freeway facility service measures will be an aid in setting transportation system and freight performance targets, a a demonstration of how they would be used for target setting is presented.