Metropolitan Washington Council of Governments

Travel time reliability is measured in various ways. Measures used in the transportation engineering field include the 90th or 95th percentile travel time, standard deviation, coefficient of variation, percent of variation, buffer index, planning time index, travel time index, skew statistic, misery index, frequency of congestion, and on-time arrival. Correlations and inconsistencies between these measures were observed on a case-by-case basis in past studies, without a full explanation or examination of the fundamental causes of such differing relationships. This paper analytically examines a number of reliability measures and explores their mathematical relationships and interdependencies. With the assumption of lognormal distributed travel times and the use of percent point function, a subset of reliability measures is expressed in relation to the shape parameter or the scale parameter of the lognormal distribution or to both. This process enables a clear understanding of the quantitative relationships and variation tendencies of different measures. Contrary to some previous studies and recommendations, this paper finds that the coefficient of variation, instead of the standard deviation, is a good proxy for several other reliability measures. The use of the average-based buffer index or average-based failure rate is not always appropriate, especially when travel time distributions are heavily skewed, in which case the median-based buffer index or failure rate is recommended.

Public-sector applications of emerging private-sector probe-based travel time and speed data have been mainly for real-time traffic operations and incident and special events management; such data have rarely been used for metropolitan planning organizations’ planning programs or processes. Using data from the I-95 Corridor Coalition Vehicle Probe Project in the National Capital Region, this paper explores the possible planning applications of such operations data, including the congestion management process, management and operations planning, air-quality conformity analysis, and travel demand modeling. The paper demonstrates several important probe data–based performance measures in metropolitan transportation planning, especially for areas without comprehensive traffic detection systems. It also summarizes the advantages and caveats of the probe data and lessons learned from the experience.

Extending the discussions in Chapter 7, this chapter revisits the topic of first mover advantages. Chapter 7 examined the existence or absence of first mover advantages in the transportation sector based on empirical observations. Examining the empirical case of London rails suggested the existence of first mover advantages in surface transportation networks, and revealed its close relationship with the spatio-temporal location of stations and network connectivity.

Washington State requires large employers in its nine most populous counties to encourage their employees to reduce commuting vehicle trips and to monitor progress by surveying employees. The monitoring requirement yields roughly 250,000 surveys every 2 years. Analysis of the survey results for 1999 and previous years estimated that the program removed an average of 18,500 vehicles from the road during the morning peak in 1999, with 12,600 of these in the Seattle metropolitan area. Information from the surveys was used to construct an origin-destination table for these trips, and the table was then used in a four-step modeling process to estimate the corridors and links that realized the greatest effect of the trip-reduction program. The modeling compared a baseline, which assumed effects of the trip-reduction program, with a case in which trips removed were added back into the trip tables. Results indicate that the trip-reduction program has measurable effects on traffic volumes and delay, both areawide and in specific corridors. The analysis is unique in having these types of data on which to draw for a metropolitan area whose modeling system can use them.