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Glossary of Mobility-Related Terms

Annual Delay per Traveler
The extra travel time for an area divided by an estimate of the number of people traveling by a motorized mode during the peak periods (6 to 9 a.m. and 4 to 7 p.m.).
Buffer Index
Measure of the reliability of travel service. Calculated as the ratio between the difference of the 95th percentile travel time and the average travel time divided by the average travel time.
Cost of Congestion
Value of travel delay and extra fuel consumed in traffic congestion.
Delay
Extra travel time compared to some standard. The standard values used in the Urban Mobility Report are 60 mph on the freeways and 35 mph on the streets.
Demand Management
Changing the time or mode of travel to have fewer vehicle trips during the peak period.
Events
Describes several causes of travel disruption including collisions, vehicle breakdowns, weather, or other unusual activities that cause traffic demand to sharply increase.
Planning Time Index
The ratio of travel time or the worst day of the month compared to the time required to make the same trip at free-flow speeds. A value of 1.8, for example, indicates a 20-minute free-flow trip requires 36 minutes during the worst peak period. (Note: Statistically the "worst day of the month" is the 95th percentile travel time).
Reliability
Variation in travel time from day to day. This concept reflects an aspect of congestion that relates to the unpredictability of travel conditions, rather than everyday slowdowns. (See Buffer Index)
Roadway Capacity
The "supply" of road transportation; measured in vehicles per hour per lane. Typical freeway values–2000 to 2200; principal arterial values–850.
Strategies
Any of the projects, programs or policies used to address congestion problems or mobility needs.
Total Delay
Total travel time delay for an urban area.
Travel Time Index
The ratio of the travel time during the peak period to the time required to make the same trip at free-flow speeds. A value of 1.3, for example, indicates a 20-minute free-flow trip requires 26 minutes during the peak period.
Wasted Fuel
Extra fuel consumed due to inefficient operation in slower stop-and-go traffic.

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