Overview
The second Strategic Highway Research Program (SHRP 2) Reliability program aims to improve trip time reliability by reducing the frequency and effects of events that cause travel times to fluctuate unpredictably. Congestion caused by unreliable, or nonrecurring, events is roughly as extensive as congestion caused by routine bottlenecks (Cambridge Systematics, Inc. 2003). Nonrecurring events such as crashes, work zones, special events, and weather disrupt normal traffic flow by causing reduced speeds, lane closures, and erratic driving maneuvers. The goals of the SHRP 2 Reliability program focus on travel time variation—the characteristic of the transportation system that can cause the driver’s current trip to take much longer than normally expected. For example, a driver must allow an hour to make a trip that normally takes 30 minutes. This transportation system characteristic is important for travelers and shippers and is a component of the congestion problem in which transportation agencies can make significant and measurable gains even as travel demand grows. Reducing delays related to reliability has the added benefit of reducing primary and secondary crashes, vehicle emissions, and fuel use, as well as other benefits.
Travel time reliability information includes cumulative data about traffic speeds and trip times that take into account historical variations from day to day and enable individuals to understand the level of variation in traffic. Unlike real-time travel time information, which provides a current snapshot of trip conditions and travel time, reliability information can be used to plan and budget in advance for a trip.
A key component in addressing the reliability issue related to urban mobility is conveying reliability-related information to system users so they can make informed decisions about their travel. The challenge for transportation professionals lies in selecting the best means of conveying that information so it is usable and effective. The goal of this research project was to examine what combination of words, numbers, and other features of user information messages, along with communications methods and technology platforms, best communicates information about travel time and reliability to travelers so that they can make optimal travel choices from their own point of view. Such choices include whether to take a trip, departure time, mode choice, and route choice.