Application of Travel Time Data and Statistics to Travel Time Reliability Analyses: Handbook and Support Materials
Chapter 1. Introduction
The purpose of this work is to provide a handbook for the application of travel time data to travel time reliability analyses. The handbook covers several topics including:
The intent of the handbook is twofold:
Defining reliability up to this point has largely been a technical exercise aimed at practitioners and researchers. For example, in the original Future Strategic Highway Research Program, reliability was defined as: “...how travel times vary over time(e.g., hour-to-hour, day-to-day).”1 This definition has persisted and formed the basis for developing reliability performance measures and analytical methods. From an analyst’s perspective, reliability is often depicted as a travel time distribution to convey variability, such as is shown in figure 1. Additional measures that describe the size and shape of the travel time distribution such as the semistandard deviation also have been used. Essentially, reliability is just a characteristic of overall congestion rather than a distinct phenomenon—how congestion varies over time.
It is generally acknowledged that the travel time distribution is used to measure reliability, but how is travel time itself defined? Travel time is measured in a variety of ways with a variety of different data (direct, indirect, and purely synthetic) and all of these methods have been used to calculate reliability. There has been almost no resource material describing these various data and methods and the implications they have on the values of reliability measures. The specific issues dealt with in this project are described below.
Example distribution of travel times taken from a segment of freeway. The distribution is skewed toward higher travel times. Superimposed on the distribution are the performance measures used to describe travel time reliability: planning time, buffer time, misery time, standard deviation, and the upper percentiles of the travel time distribution. The point here is that all of the measures used to describe travel time reliability are developed from the distribution of travel times for a facility or trip.
Trip-Based Travel Time Reliability
Because of the nature of the data that have been available, nearly all reliability reporting is based on the facility perspective. The data measurements used relate to the performance of a facility, not an end-to-end trip as made by travelers. Trip performance can be synthesized from facility-based data using the virtual probe method, but how well this method represents actual vehicle travel times has not been determined.3
A comprehensive mobility measurement program will involve using both trip- and facility-based measures because they both inform analysis about the nature of mobility in a region:
Data from vendors are now becoming available that allow trip-based measures to be developed; these data track the location and time of individual vehicles and are described throughout this handbook. However, trip-based reliability measurement poses its own challenges. Facility measurement describes the nature of congestion to which travelers are exposed. When using these data, analysts are left to decide the origin and destination of a trip. Trip measurement includes factors in addition to congestion exposure—how travelers interact with the entire landscape. Regarding trips, travelers are generally free to change departure times and routes and, in some cases, destinations and modes as well. In this project, the research team fixed the origins and destinations to compare trajectory data to facility-based data.
Over time, however, trip purposes and destinations change, resulting in multiple definitions of what a “trip” is even though it may be measured with the same measure. For example, a work trip could have the same start time and route as well as exclusively use a car every day. Alternately, these factors could vary to different degrees. Measuring a true trip from the traveler’s perspective entails measuring a variety of factors, many of which are beyond the control of transportation agencies. Finally, measuring trip performance can be viewed as how participants (travelers and businesses) adapt to the landscape. This adaptation no doubt includes congestion avoidance (e.g., selecting origins and destinations to minimize congestion exposure) and associated costs, which are not captured in trip-based measures.
Standard Processing Procedures
Standard processing procedures for calculating performance measures from high-resolution data do not exist. Analysts use different methods for performing quality control (QC), imputation/handling of missing data, aggregating data, and computing measures, resulting in different values for performance measures created from the same data.
Because little detailed data collected under rigorous controls exist for comparison, QC procedures for travel time data are primitive. For freeway detectors, where volumes and speed measurements exist, checks can be made against traffic flow parameters but deciding how far astray a value should be before it is considered erroneous is problematic. For vehicle probe data, the situation is even more restrictive. Cross-checking travel time data against disruption data (i.e., weather, incidents, and work zones) would be a way of verifying that low speeds are legitimate, but this project did not deal with data from these other data sources.
1 Transportation Research Board of the National Academies of Sciences, Engineering, and Medicine. 2003. NCHRP Report 510: Interim Planning for a Future Strategic Highway Research Program. Washington, DC: National Academy of Sciences. [Return to note 1]
2 Transportation Research Board of the National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual, Transportation Research Board. Report No. S2-L08-RW-1. Washington, DC: Transportation Research Board. [Return to note 2]
United States Department of Transportation - Federal Highway Administration