How and Where Volume, Speed, and Reliability Profiles were Measured
Volume, speed, and reliability were analyzed for four central freeway locations in the Seattle area “rectangle” bounded by I-5, SR 520, I-405, and I-90, plus one location on SR 167. The selected locations were downtown Seattle on I-5 at University Street, downtown Bellevue on I-405 at NE 14th Street, SR 520 at 84th Avenue NE, I-90 on the floating bridge, and SR 167 a mile south of the I-405 interchange. While these five sites are not representative of all freeway sections, they provide considerable insight into the region’s freeway system performance.
Each grapc shows three measures of system performance for one direction and type of lane at the selected freeway location. These measures include the following:
- Average weekday traffic volume profile at a location, by time of day
- Average weekday speed at a location, by time of day
- Frequency of significant congestion at a location, by time of day for weekdays
Average Weekday Traffic Volume Profile at a Location, by Time of Day. These data come from loop detectors placed by WSDOT in the mainline freeway lanes to support the Department’s ramp metering and incident response systems. Loop detectors report vehicle volumes and the percentage of time that a vehicle is present over that loop. These statistics are reported every 20 seconds continuously throughout the year. The graphs can be converted into measures of traffic volume and speed and summarized over time.
Because both the magnitude and timing of volumes during the day are important considerations in trying to understand freeway performance, an average 24-hour weekday traffic profile was estimated at selected freeway sites. Vehicle volumes were estimated at 5-minute intervals over an average 24-hour weekday at each site; these volumes were then adjusted to an equivalent per-lane hourly rate (vehicles per lane per hour, or vplph) to allow direct comparisons between sites with different numbers of lanes.
Average Speed at a Location, by Time of Day. Because traffic volumes are affected by the speed that vehicles can travel (and vice versa), the average speed observed at each time of day was also estimated throughout the day for the selected sites. Average speeds are displayed by color coding of the vehicle volume line.
Frequency of Significant Congestion at a Location, by Time of Day. Average conditions do not represent the condition that always occurs; on some days traffic conditions will be worse than average, and on other days conditions will be better. In fact, freeways tend to operate in one of three conditions,
- free flowing
- heavy traffic moving, but moving below the speed limit
- “badly congested” or unstable (stop and go) flow
A good measure of how motorists perceive traffic is how often a freeway exhibits “bad” or unstable flow. Congestion frequency is measured by the likelihood that significantly congested traffic will be encountered at a location at a given time of day. For example, a congestion frequency of 75 percent at 5:00 PM indicates that there is a 75 percent chance of encountering congested conditions (stop and go, or unstable flow) at a particular location in a given direction of travel at 5:00 PM. |