Freeway Performance

HOV Lane Volume, Speed, Reliability Conditions (by location) (draft 8 November 2005)

How and Where HOV Lane Volume, Speed and Reliability Conditions Were Measured

Examining the operation of HOV lanes at specific locations provides detail about HOV traffic performance. These graphics report several key performance statistics at representative locations along each HOV corridor.  The principal measures used to evaluate HOV performance at a particular site include the following:

Average vehicle volume at a location, by time of day. Vehicle volumes calculated at 5-minute intervals over a 24-hour weekday, and averaged over a full year at a given site. These volumes are then adjusted to a "per lane" hourly rate, or vehicles per lane per hour (VPLPH) to allow direct comparison between sites with varying numbers of lanes.

Average speed at a location, by time of day. Weekday speed for a location was calculated at 5-minute intervals over a 24-hour weekday and averaged over a full year.  The graphic only displays whether that average speed is faster or slower than the 45 mph performance standard adopted by the region.

Percentage of days during which the average speed is less than 45 mph at a location. The percentage of weekdays during which vehicles in the HOV lane at a specific time period and a specific location travel at less than 45 mph. This measure helps show how "reliable" a given facility is.

Data were collected from 11 sites.  

I-5 north
of the Seattle CBD

I-5 @ 112th SE – Everett (near Everett)
I-5 @ NE 145th St (near Northgate)

I-405 @ NE 85th St (near Kirkland)
I-405 @ SE 52nd S. (near Newcastle)

How to Read the Average Weekday Volume, Speed, and Reliability Conditions Graphs

Site-specific HOV lane performance is illustrated with a graph that combines HOV lane volume by time of day, and the frequency with which the HOV lane at that location fails to average 45 mph.
 [Figure 3-18. (Example) Average Volumes, Speed, and Speed Reliability Conditions Graph: Southbound on I-5 NE 137th St.]

The example [embed fig. 3-18] illustrates performance for the southbound I-5 HOV lane at NE 137th St. near Northgate. It shows average volumes and travel speed conditions from 4:00 AM to 12:00 midnight. The horizontal axis represents time of day. (Usually from midnight to midnight, but for this example only twenty hours are actually shown.) The line shows the expected traffic volume, and is measured with the left vertical axis in units of vehicles per lane per hour. The color of the line indicates the expected speed of vehicles in the HOV lane on the average day:

The column graph is read from the right vertical axis, which shows the frequency of congestion (speeds below 45 mph) at this location. In this example, HOV volumes exceeds 1,400+ VPLPH during the AM peak period. Travelers can expect to travel faster than 45 mph throughout the day, except between 7:00 AM and 8:00 AM, when they have approximately a 30 percent chance of encountering speeds of less than 45 mph.

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.