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.