Freeway Performance

GP vs HOV Travel time (by corridor) (draft 7 Nov 2005)

 

How and Where Travel Time Profiles Were Measured

For each of 22 trips (11 routes, with facility performance shown in both directions), data were used to estimate three measures:

  • average travel time
  • 95th percentile travel time (i.e., a measurement that indicates that nineteen times out of twenty— 95 percent of the time—a trip’s travel time will be less than a certain number of minutes)
  • the likelihood of a ‘slow’ trip, defined to be an average overall trip speed on the freeway of less than approximately 35 mph.

These measures are all summarized graphically as a function of the trip start time and trip direction. All trips assume freeway-only routes on general purpose mainline lanes and freeway-to-freeway ramps during weekdays. On- and off-ramps, HOV lanes, and express/reversible lanes are not included.

Note that the three travel time-related performance measures (average trip time, 95th percentile trip time, and likelihood of slow trips) are estimates, and are used for comparative purposes rather than as absolute values. The travel times reflect “average” freeway conditions on these routes at different times of day; a specific travel time on a given day may differ from these estimates because of an individual’s driving style and vehicle characteristics, as well as that day’s traffic levels, congestion patterns, and weather conditions. Note also that the travel time estimates reflect the cumulative effect of all congestion along a given route; the overall trip time may be affected by one large section of congestion, or a number of smaller slowdowns along the way. It is therefore also useful to compare the travel time patterns with the geographic patterns of congestion seen in the traffic congestion maps to gain a better understanding of the association between specific bottleneck locations and trip travel time.
  I-5

Everett (SR 526) — Seattle CBD (University St)
SeaTac/Federal Way (S 272nd St) to Seattle CBD

  I-405

Mountlake Terrace (I-5 interchange) — Bellevue CBD (NE 8th St)
Tukwila (I-5 interchange) — Bellevue CBD (NE 8th St)
  SR 167 Auburn (SR 18) — Renton (I-405 interchange)
  I-90/I-5 Issaquah (Front St) — Seattle CBD (University St.)
  SR 520/I-5 Redmond (NE 60th) — Seattle CBD (University St)
  I-405/SR 520/I-5 Bellevue CBD (NE 8th) — Seattle CBD (University St)
  SR 520/I-405 Redmond (NE 60th) — Bellevue CBD (NE 8th)
  I-90/I-405

Issaquah (Front St) — Bellevue CBD (NE 8th)
  SR 524 Bothell to Bellevue

How to Read Travel Time Profile Graphs

The example [embed (Figure 3.18) shows an 8-hour slice of a typical travel time graph, in this case for westbound SR 520 general purpose lanes, for a trip from Redmond Way to I-5. The estimated travel time is a function of the time of day that the traveler begins the trip, shown along the horizontal axis. The green line represents the average travel time, measured with the left vertical axis, which in this example varies from about 17 minutes for a person leaving Redmond at midday, to about 27 minutes during the evening commute.

The red line represents the 95th percentile travel time, which in this example can reach about 41 minutes in the evening peak, meaning that nineteen times out of twenty (i.e., 95 percent of the time) the trip’s travel time will be 41 minutes or less. The 95th percentile travel time measure describes those conditions experienced on a “bad day” for that trip. The degree to which a trip’s 95th percentile travel time differs from its average travel time indicates the variability of conditions routinely experienced for that trip. So, for someone leaving Redmond at 5:00 PM the trip ranges from 27 minutes, on average, to approximately 41 minutes or less 95 percent of the time.

Superimposed on the two travel time lines is a column graph, measured along the right vertical axis, that illustrates the estimated frequency of congestion on a given trip or route. Congestion frequency is measured by the likelihood that the average trip speed will be below 35 mph for a given trip start time. On the example trip, the congestion frequency of almost 61 percent at 5:30 PM indicates that there is approximately a 61 percent chance that the average overall trip speed will be less than 35 mph when the trip starts at 5:30 PM.

 
 

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.
 
Revised 23-Nov-2005