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CHAPTER 17
CASE STUDY 8: TRANSIT IMPROVEMENT
Case Study 8 involves doubling the frequency of bus line (However, one option for overcoming this limitation of the
7B (PSRC Model Line 4007), providing bus service on database would have been to take the AM peak-hour impacts
Broadway between Downtown Seattle and South Rainier of the project and double them to approximate the combined
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Beach. This bus line extends for 23.55 route-miles (see Fig- AM and PM peak-hour impacts.)
ure 14).
17.2 CASE STUDY RESULTS
17.1 APPLICATION
The NCHRP 25-21 methodology was used to compute the
Before improvement, the bus service operated at 30-minute impacts of the traffic-flow improvement on the 2020 base case
peak-hour headways, with a mean speed of 15 mph. After trip tables by time period (AM peak, PM peak, and off peak)
improvement, the peak-hour headway was cut from 30 min- and by mode (SOV, HOV, and transit). The revised trip tables
utes to 15 minutes. Note that the service improvements were were then reassigned to the improved network to determine the
meant to apply to both the AM peak and PM peak hours. impact on VMT, VHT, and emissions. Facility-specific results
However, in the PSRC model, transit trips outside of the AM were not computed. The regional impacts of the transit ser-
peak hour are not assigned to specific transit lines. Conse- vice improvements were found to be negligible within the
quently, the test of the PM peak-hour service improvements precision of the reported results. There were no significant
could not be made with the available PSRC model database. differences in VMT or emissions.
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Seattle
Project
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Figure 14. Case Study 8: Transit improvement.
