Vehicle Travel Time Reliability: Percent change in the day-to-day variability of vehicle travel time

Status

The Buffer Time Index (BTI) measures how reliable travel times are along a roadway, with a lower score indicating less  day-to-day variability and thus greater reliability. This measure indicates that travel times along Austin arterial roadways are consistent and the BTI percentage is within the goal threshold of 25%.

Trending

In recent years, the BTI percentage for Austin arterial roadways has remained very consistent at approximately 8%, which is within the target threshold of 25%. A steady BTI can be interpreted as travel time reliability is remaining relatively constant during peak periods even if travel times are increasing as population increases (see percent change in average vehicle travel time along major city corridors).
Note: To see the underlying data for this chart, please select the "View Source Data" link.

Additional Measure Insights

City engineers and technicians work to address not only recurring traffic congestion, but non-recurring congestion related to crashes, special events, construction, and other irregularities. By adjusting signal timing in real time to address these changing conditions, arterial travel times are made more consistent. By working to provide a more reliable travel time along each corridor, road users can better plan their trips and make more confident travel decisions.

Measure Details and Definition

1) Definition:  Anonymous travel time data is provided by a third-party vendor, INRIX. This data measures speed and travel time along Austin streets. Travel times along street segments are measured over the course of each year and are analyzed along Austin’s arterial roadways.

The Buffer Time Index (BTI) specifically measures how variable travel times are along roadways. A high BTI would indicate that the travel times change a lot. A low BTI would indicate that the travel times are very consistent. BTI indicates how much longer the 95% travel time (95% of all trips occur within this amount of time or less, also called the Planning Time) is than the average travel time (from 6 a.m. to 8 p.m. each weekday) and is reported as a proportion of average travel time. An example BTI percentage of 20% on a roadway with an average travel time of 10 minutes would indicate that 95% of all the trips along a roadway occur in 12 minutes or less. So, if you wanted to be on time to your destination 95% of the time you travel this roadway, you’d plan for it to take 12 minutes which is only 20% more than the average travel time.

2) Calculation method: The Buffer Time Index (BTI) measures travel time reliability. BTI is calculated using the following equation:

(95% Travel Time - Average Travel Time) / Average Travel Time 

This measure is calculated for all critical arterial corridors during the hours of 6 a.m. - 8 p.m. on weekdays for each year. 

3) Data Collection Process: This data is provided by a third-party vendor, INRIX, which reports speed and travel time data along roadways. This data is recorded from anonymized data from smart devices used during travel along corridors shown on this web map. Calculations were adjusted to remove missing or incomplete data within the INRIX dataset for 2017 for the following corridors: Burleson Road, Metric Boulevard, and South Pleasant Valley Road.

In 2021, the INRIX segments for which this metric was calculated on was adjusted, resulting in a lower BTI as compared to previous years. In order to maintain historical integrity, previous years’ data was not altered. For this reason, data from 2020 and beyond is significantly different than the previous years.

4) Measure Target Calculation: This target was selected based on general understanding of traveler expectations, engineering judgment, and technical expertise. The Federal Highway Administration found that the average BTI percentage for urban freeways in the U.S. is 35%. We expect a lower threshold for arterial roadways. Therefore, we chose 25% as the target goal.

5) Frequency Measure is Reported: Annually (Calendar Year)

Date page was last updated: July 2022