Disseminating Traveler Information on Travel Time Reliability
CHAPTER 1. OVERVIEW
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.
THE CONCEPT OF TRAVEL TIME RELIABILITY
Transportation professionals typically describe Travel Time Reliability (TTR) in terms of historical average travel times calculated over periods of a year or longer, as shown in Figure 1. A typical definition for TTR would be the following:
The consistency or dependability in travel times, as measured from day to day and/or across different times of the day.(1)
Figure 1. Graphic. Average travel time used by professionals.(1)
Source: https://ops.fhwa.dot.gov/publications/tt_reliability/brochure/index.htm#fig1m
However, most system users do not experience the same average travel time each day. Rather, they experience and remember something much different than the average throughout a year of commute trips (see Figure 2). Their travel times vary greatly from day to day, and they are more likely to remember a few bad days during which they suffered longer-than-typical travel times than a good day or a typical day.(1) Research has shown that TTR information can provide transportation system users with a more complete picture of expected travel times along a particular route. The challenge is how to communicate that reliability information effectively to system users so that they understand it clearly and respond appropriately to improve their trip experience.
Figure 2. Graphic. Traveler travel time experiences.(1)
Source: https://ops.fhwa.dot.gov/publications/tt_reliability/brochure/index.htm#fig1m
Real-time travel time messages have been in use in the U.S. since traffic monitoring and integration systems became reliable more than a decade ago. Typically, the most commonly used media for these messages are Dynamic Message Signs (DMSs) and transportation agency websites. However, the widespread use of cell phones and other mobile devices has resulted in an explosion of new mechanisms for transportation agencies and third-party providers to offer real-time updates on transportation conditions and options via e-mails, text messages, mobile application push notifications, and Twitter feeds.(2)
Real-time travel time estimates are most often provided for a particular roadway segment, facility, or transit route based on recent travel speeds or conditions. Some agencies also provide travel time comparisons among two or more routes/roadways to help travelers make decisions about the route or transportation mode to take. More recently, agencies are beginning to provide TTR information for facilities: the likelihood that the estimated travel time for a particular trip or trip segment will be dependable.
As discussed previously, travel time variability describes how a traveler's trip will vary from what is normally expected. One measure of travel time variability is travel time reliability. TTR information includes static data about traffic speeds or trip times that capture historic variations from day to day or over a specified period of time and enable individuals to understand the level of variability in traffic.(2) 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.
Typically, travelers might want to access TTR information for such purposes as: (a) trip planning for habitual trips such as commutes, or when new to an area and unfamiliar with routes and typical travel times; (b) pre-trip planning immediately prior to departure, to make decisions about departure time and/or mode based on real-time and historical travel time trends, especially if traveling at a time or to a destination that is not typical; and (c) changing their trip while in progress prior to a route or mode choice point (again based on both real-time and historical information regarding particular routes at particular times of the day).(1)
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. Furthermore, transportation agencies can use TTR information to assess system performance and use that information to inform local decision makers about congestion challenges and/or needs in the region. The challenge for transportation professionals lies in selecting the best means of conveying that information so that it is usable and effective to the traveler and other stakeholders.
