Exploring Travel Time Distribution and Variability Patterns Using Probe Vehicle Data: Case Study in Beijing

Table 1

Travel time reliability measures.

Category	Measure	Definition
Probability index	Variance	The expected value of the square of the deviations of a random variable from its mean value
	Standard deviation	The square root of the variance
	Coefficient of variation	Standard deviation divided by the average travel time
Statistical index	th percentile	th percentile of the travel time distribution
	Skewness statistic	The ratio of the difference between 90th and 50th percentile travel time to the difference between 50th and 10th percentile
	Width statistic	The ratio of the difference between 90th and 10th percentile travel time to the 50th percentile travel time
	Travel time index	The ratio of the average travel time to free-flow travel time
	Planning time index	The ratio of 50th percentile travel time to free-flow travel time
Buffer time type index	Buffer time index	The ratio of the difference between 95th percentile and the average travel time to the average travel time
	Failure/on-time performance	Percent of trips with travel time less than, for example, , , and so on
	Frequency of congestion	Percent of time that the travel time is larger than double the free-flow travel time
	Misery index	The average of the highest five percent of travel times divided by the free-flow travel time