The OKI region’s quality of life and economic competitiveness are closely related to the degree to which the transportation system is able to provide an acceptable level of mobility. Congestion is the level at which transportation system performance is no longer acceptable due to traffic interference. This traffic interference may be recurring or non-recurring congestion. Recurring congestion is caused by consistently excessive travel demand as compared to available roadway capacity. Sometimes, poor signal timings, poor access management and roadway geometric deficiencies contribute to reduced capacity. Non-recurring congestion occurs due to traffic incidents, adverse weather or road construction. The relative severity of travel congestion is measured by travel time, speed and the calculated measure called travel time index.
When planning a trip, it’s a good idea to build in some extra time as a cushion against the effects of heavier-than-normal traffic or other unforeseen events. Transportation planners have a term for this cushion: buffer index. The buffer index measures the variability of travel time along a corridor.
The following map and charts show the buffer index during AM and PM peak periods. It represents the extra time (buffer) travelers should add to their average travel time when planning trips. This is the extra time between the average travel time and near-worst case travel time (95th percentile). The buffer index is expressed as a percentage and its value increases as reliability gets worse ((95% Travel Time – Average Travel Time) / Average Travel Time).
For example, a buffer index of .4 (40 percent) means that, for a 20-minute average travel time, a traveler should budget an additional 8 minutes (20 minutes × 40 percent = 8 minutes) to ensure on-time arrival most of the time.
Data Source: INRIX, Inc. Performance Summaries for Selected Corridors during weekdays in 2014.
Planning Time: How much total time in minutes a traveler should allow to ensure on-time arrival (Average Travel Time + Buffer Time).