and education of those responsible for planning, programming, operating, and maintaining the highway system. Political leaders need to understand how highway operations, as well as new construction and maintenance, affect congestion and consequently how suboptimal resource allocation decisions can hamper a comprehensive approach to congestion mitigation.
SHRP 2 addresses congestion in two strategic focus areas. The Capacity focus area, discussed in the following chapter, addresses the need for new physical capacity to deal with recurring congestion and explicitly includes economic, environmental, and pricing aspects of the issue. The Reliability focus area, described here, addresses a particular operational characteristic of highway systems that is related to nonrecurring congestion: travel time reliability. From the highway user’s perspective, travel time reliability means the extent to which one can depend on completing a given trip within a consistent, predictable length of time. More specifically, travel time reliability is the probability or percent of time that a person or goods shipment will arrive on time (or within a time window) for a particular type of trip, departure time, origin and destination, and environmental setting. When travel times are not reliable, travelers frequently find themselves late unless they build additional buffer time into their trips. For example, one may leave earlier to ensure arriving on time for an important appointment or to catch a flight. This buffer time could have been spent on other activities and could actually be spent waiting at one’s destination if the time was not needed in the end.
SHRP 2 research is aimed at improving travel time reliability by addressing the nonrecurring portion of the congestion problem. The relatively unpredictable events that are responsible for nonrecurring congestion can be categorized as follows:
Traffic incidents: This category covers a number of different types of events. Crashes are a common example; when a crash occurs on a roadway, at least one lane of traffic is typically blocked by the vehicles involved in the crash. Often, additional lanes are blocked by police, fire, and emergency medical vehicles and equipment. Even if the crash occurs off the road or has been moved to the shoulder, through traffic must slow down for safety