Freeway Performance

HOV Speed and Reliability (by corridor) (draft 23 November 2005)

How and Where HOV Speed and Reliability Were Measured

Speed. Because the regional policy standard for HOV lane performance requires an average speed of 45 mph or better, 90 percent of the time during the peak hour, the 90th percentile weekday HOV lane speeds were estimated for a range of trip start times throughout an average 24-hour weekday.  This measurement indicates that nine times out of ten, a vehicle will travel at a particular speed or faster.  Average speed is computed for the entire length of the trip along the HOV corridor.

Reliability. In contrast to the 90th percentile average travel speed, this measurement indicates the percentage of weekdays that the average trip speed will be below 45 mph for a given trip start time.  This measure indicates how frequently an HOV lane fails the 45 mph standard adopted for Puget Sound freeways.

Speed and reliability were measured along the following corridors:

How to Read the HOV Speed and Reliability Graphs

[Figure 3-1. ( Example) Average Weekday HOV Speed and Reliability Graph: I-5 North of the Seattle CBD, Northbound from NE 130th St. to Alderwood]

The example [embed Fig. 3-1] is an 8-hour slice of a speed and reliability graph for the northbound HOV lane on I-5 near Northgate. Both of these measures depend on the time of day the traveler leaves. The starting time of a trip is shown along the horizontal axis, which runs from midnight to midnight. (In this example, however, only the hours from 1:00 PM to 9:00 PM are shown.) The line graph near the top of the figure represents the 90th percentile average speed for HOVs using I-5 from Northgate to Alderwood. It is measured with the left vertical axis. It indicates that HOV lane vehicles travel at a speed below 45 mph (about 44 mph) between 4:00 PM and 5:20 PM.  Thus, this particular corridor is not meeting the regional performance guidelines.  As a result, the specific causes of the failure are now under study by WSDOT, and potential solutions to those problems will be considered along with other needs and deficiencies as part of the regional project selection and prioritization process.

The column graph at the bottom of the figure is measured with the right vertical axis. It illustrates the reliability of this HOV trip. It shows that the percentage of time travel speed for this trip (leaving at the time indicated) averages less than 45 mph. In this example, between 4:00 and 5:00 PM, the reliability of this trip is about 10 percent. This can be interpreted as a "a carpool on this route will average less than 45 mph once every two weeks when making this trip in the evening rush hour."

Disclaimer. The contents of this Web page reflect the views of the authors/researchers, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Washington State Transportation Commission, Department of Transportation, or the Federal Highway Administration. The information presented does not constitute a standard, specification, or regulation.