Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Appendices
The Issue
State Road 520 (the Evergreen Point Floating Bridge) is an east-west facility that connects I-5 in
Seattle with Bellevue, Washington and other suburbs to the east. This facility (shown in Figure
D.1) has a cross-section of two lanes per direction. The ramp meters for traffic entering eastbound
State Road 520 from Montlake and Lake Washington Boulevards have operated during the
evening commuting period since 1986. Since then, eastbound traffic conditions (in what had
traditionally been the reverse commute direction) during the 6:00 a.m. to 9:00 a.m. peak period
have worsened. This has occurred due to the population growth of Bellevue and due to business
development (e.g., Microsoft) east of Lake Washington. Until late-2001, however, eastbound onramp
meters were not active during the morning commuting period. In an effort to alleviate
increasingly heavy morning congestion and to reduce merge-related accidents, the Washington
State DOT decided to lift the time restrictions for the operation of the eastbound on-ramp meters.
Since August 6, 2001, these ramp meters have operated during weekday morning commuting
times.
Figure D.1 – The State Road 520 (Evergreen Point Floating Bridge) Study Area
Source: Google Map
Problem Statement
This sample problem presents a methodology for determining how the activation of these two onramp
meters for eastbound State Road 520 traffic has affected travel-time reliability during the
morning peak period. Six months of weekday 5 minute-interval travel time data from before and
after the initiation of morning ramp metering were analyzed. Travel times during the before period
(January 1, 2001 to June 29, 2001) were compared with travel times during the after period
(January 1, 2002 to June 28, 2002). This comparison included travel-time measurements taken
during each weekday in a comparable six-month period before and after activation of these two onramp
meters to minimize the effects of seasonal variation.
Eastbound travel times were measured on weekdays between 6 a.m. and 9 a.m. Speeds were then
calculated based on the approximately four-mile length of State Road 520 from I-5 to the east side
SAMPLE PROBLEM-QUANTIFYING THE ECONOMIC BENEFIT OF IMPROVING TRAVEL-TIME RELIABILITY Page D-2