Disseminating Traveler Information on Travel Time Reliability
EXECUTIVE SUMMARY
Travel time variability is that characteristic of the transportation system that means a traveler's trip will vary from what is normally expected and will potentially take longer than planned. This transportation system characteristic is important for travelers and shippers. It also is a component of the congestion problem for which transportation agencies can make significant and measurable gains, even as travel demand grows. By providing the means to help travelers make travel choices that take into account travel time variability, agencies have the potential to reduce roadway congestion. This reduced congestion has the added benefit of reducing primary and secondary crashes, vehicle emissions, and fuel use, as well as yielding other benefits.
One measure of travel time variability is Travel Time Reliability (TTR). Transportation professionals describe TTR in terms of historical average travel times calculated over periods of a year or longer. TTR information includes static data about traffic speeds or trip times that capture historic variations from day to day and enable individuals to understand the level of variability in traffic. TTR information is considerably different from real-time travel time information and may have different uses. A traveler can use reliability information to plan and budget in advance for a trip rather than simply react to current conditions. A key component to addressing the reliability issue related to urban mobility is conveying this TTR information to system users so that they can make informed decisions about their travel. The challenge for transportation professionals lies in selecting the best means of conveying information so that it is usable and effective to the traveler and other stakeholders.
The Strategic Highway Research Program 2 (SHRP2) Reliability Project L14 established a preliminary set of suggested terminology and guidelines for conveying TTR information to road users so that they may make optimal travel choices from their point of view, such as whether to take a trip or not, departure time, mode choice, and/or route choice. Specifically, a Lexicon of phrases was developed for each of eight TTR terms, which contained detailed guidelines for TTR information that would most likely be understood and used by travelers. To help agencies and other transportation-related entities better deploy and use the recommended Lexicon terminology, a field study was conducted to test the phrases to demonstrate the technical and institutional feasibility of their use and determine the potential costs and benefits of using these products of L14. Two Lexicon assemblies and three dissemination platforms were tested in the field study.
Overall, the field study found that the particular Lexicon phrases tested performed similarly. Only one survey question resulted in statistically significant differences between the Lexicon phrases. The biggest finding of the field study was that dissemination of TTR information via the 511 information channel was less preferred than via the Web or App access channels. In a few instances, the App access demonstrated superior responses over both the Web and 511 access.
Agencies considering dissemination of TTR information to their system users should consider various scenarios in which travelers and other stakeholders might find that information valuable. Some of these scenarios are individual trip planning for familiar trips, individual trip planning for unfamiliar trips, individual trip changes, alternate route comparisons, employment center locations, and overall system reliability. Agencies also will need to obtain historical traffic datasets as a source for determining the TTR calculations. These calculations can then be utilized by information dissemination platforms so that travelers can make informed decisions about their trip.
If an agency determines that TTR information will be valuable to its system users, it is important to clearly explain to them the difference between real-time information and TTR information. It is highly likely that system users will be familiar with existing real-time traveler information for the region from the plethora of sources available to them across providers and information dissemination platforms. They may not intrinsically understand what TTR information is telling them, so an explanation is important for comprehension. Furthermore, providing examples of how travelers might use TTR information for trip planning (e.g., unfamiliar trips, familiar trips at unusual times, etc.) may help increase awareness and overall use of the information by travelers. If the transportation agency already provides real-time traveler information, then comparing the two types of information in a side-by-side comparison might help with comprehension and usage.
