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57 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 USER'S GUIDE 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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58 Seattle Project USER'S GUIDE Figure 14. Case Study 8: Transit improvement.