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45 C h a p t e r 7 All the planned scenarios went through the complete modeling process depicted in Figure 6.10. A video showing the simulation of DynusT is available at http://youtube/ FslkZE_ztAs. 7.1 route Options The route options displayed in Figure 7.1 illustrate route utilization under different reliability considerations. When reliability is not considered for a TigardâPortland central business district (CBD) O-D pair, a longer and less reliable route (blue route in Figure 7.1a) is considered in the route set. When a reliability measure is considered, a set of shorter routes considered by users is shown. That set excludes the blue route. A similar route set pattern is observed for the Tualatinâ Portland CBD O-D pair (Figure 7.2). The case considering reliability measures includes fewer routes as it omits longer and less robust routes. Such assessments were confirmed by local commuters with actual experience with the corridor. 7.2 Impact of reliability on perceived travel times 7.2.1 Tigard and Portland CBD Further details from skims prompted some insights regarding the composition of travel time equivalents (TTEs). For the same TigardâPortland CBD O-D pair in the baseline with reliability case, the total TTE was 48 min and travel time was 41 min, indicating a 7 min TTE of reliability. In the case of BRT operating in an exclusive ROW via adding a new lane, with reliability the TTE was 46 min and travel time was 40 min, meaning a 6 min TTE of reliability. Despite a 1-min difference, both cases exhibit comparable TTEs of reliability. Figure 7.3 shows the peak-period TTEs for Tigard and the Portland CBD. 7.3 Impact of reliability on transit Mode Shares Considering reliability generally leads to a slightly increased transit ridership share compared to a no-reliability case, as shown in Figure 7.4. Comparing three no-reliability sce- narios (the baseline case, BRT in an exclusive ROW via an added lane, and VMS + BRT in an exclusive ROW via an added lane) revealed a generally increased ridership share for BRT (increase from 4.15% in the baseline case to 4.5% in the BRT in the exclusive ROW via an added lane case), but a slightly decreased ridership share for the VMS + BRT case compared to BRT. The same scenarios analyzed by including a reliability measure revealed a similar trend across the three scenarios, but the reliability cases generally concluded with an approxi- mately 1% increase in ridership compared to the no-reliability cases. This difference is due primarily to the fact that includ- ing a reliability measure increases the impedance of auto- mobile traffic. However, such reliability-related impedance is minimal for BRT, because BRT travels on the dedicated ROW and hence is less likely to be affected by congestion on the automobile network. The ridership differences among sce- narios were generally less than 1.0%. The differences were likely to be subject to the randomness of the mesoscopic sim- ulation. Nonetheless, considering that the total share of transit in the study corridor is only about 4%, the range of differences appears to be intuitive. 7.4 transit Mode Shares by Scenarios The transit market shares under the four scenarios displayed in Figure 7.5 appear to be intuitive, with transit accounting for about a 4.33% mode share in the baseline case, 4.74% in the BRT in exclusive ROW via an added lane case, 4.88% in the BRT in exclusive ROW via a take a lane case, and 4.65% in Scenario Analysis
46 the BRT in mixed traffic case. One could understand these differences from the standpoint of traffic conditions for both automobile and BRT in each scenario. In the BRT in exclusive ROW via added lane case, the dedi- cated traffic lane improved both the travel time and the reli- ability for BRT, thus leading to an increase of market share. In the BRT in exclusive ROW via lane removal case, which takes away one automobile lane, the roadway capacity for automobile traffic is significantly reduced, resulting in a higher level of con- gestion and thus reducing the market share of automobiles. 7.5 Impact of Variable Message Signs on transit Mode Shares The general function of VMSs is to provide warning and/ or guidance about upstream congestion to alert automobile drivers to the situation and allow them an opportunity to divert to another corridor. This function is particularly useful in the portion of the study area where Barbur Boulevard runs parallel to I-5. When congestion occurs on either highway, the installed VMS helps balance the traffic load on both corridors. This load- balancing mechanism helps alleviate traffic congestion and consequently increases the automobile market share due to improved traffic conditions. As shown in Figure 7.6, the transit market share slightly decreases in all three shown scenarios because VMSs improve traffic conditions for automobiles. This example highlights the market interrelationship between differ- ent modes and traffic management strategies. Although seem- ingly undesirable from a transit operation standpoint, the systemwide total benefit of including both BRT and VMS is properly captured when reliability is incorporated. The effects of reliability in scenario analysis are discussed in Section 7.6. 7.6 Impact of reliability on Scenario analysis Another method for understanding how incorporating reliability measures could affect the scenario analysis out- come is to examine the percentage improvement or change of (a) Without Reliability (b) With Reliability Figure 7.1. Route choice options between Portland central business district (CBD) and Tigard.
47 person-based measures of effectiveness (MoEs) [i.e., average travel time, average vehicle miles traveled (VMT), and average delay] across scenarios for cases both with and without reliabil- ity. Examining the scenario outcomes based on person-based (in lieu of automobile mode only) MoEs for the entire corridor is the proper way to depict the performance of the study corri- dor. This approach is of particular importance because most of the studied scenarios are related to BRT operations, and the auto-only MoE does not reflect the actual performance of the scenarios of interest. Figure 7.7 and Figure 7.8, respectively, illustrate the without reliability and with reliability cases on southbound Barbur Boulevard. The charts on the left-hand side of each figure show the MoE values for each of the three sce- narios, and the right-hand charts show the improvement with respect to the baseline case in terms of percentage reduction. The chart without considering reliability (Figure 7.7) shows that the BRT strategy improved on the baseline case by about 2% in average personal travel time and 7.5% in average delay. For the BRT + VMS case, the improve- ment in average personal travel time was increased to 4.7% from 2% in the BRT-only case, but the improvement in average delay shrank to 4.7% from 7.5% in the BRT- only case. Compared to the case without reliability, the main observed difference in the case with reliability on southbound Barbur Boulevard was the increased benefit measures (Figure 7.8). The reduction was 11% for average personal travel time and 11.5% for average delay for the BRT scenario, and 10% and 18.5% improvement for average travel time and delay, respec- tively, for the BRT + VMS case. Similar conclusions can be observed for the I-5 corridor (Figures 7.9 and 7.10). Taking southbound as an example, considering the reliability measures increased the benefit of BRT by 15% and BRT + VMS by 20% with respect to average personal travel time and delay. These values are considerably higher than those in the case without consid- ering reliability (2.3% and 5.4%, respectively, for average travel time for BRT and BRT + VMS and 8.5% and 3.6%, (a) Without Reliability (b) With Reliability Figure 7.2. Route choice options between Portland CBD and Tualatin.
48 Figure 7.3. Peak-period travel time equivalents between Tigard and Portland CBD.
49 Figure 7.4. IntraâSouthwest Corridor transit percentagesâ comparing scenarios with and without reliability. Figure 7.5. IntraâSouthwest Corridor transit percentagesâ all scenarios with reliability.
50 Figure 7.6. IntraâSouthwest Corridor transit percentagesâcomparing all scenarios with reliability, with and without vehicle message signs. Figure 7.7. Southbound Barbur Boulevard measure of effectiveness (MoE) comparisonâwithout reliability. Figure 7.8. Southbound Barbur Boulevard measure of effectiveness (MoE) comparisonâwith reliability.
51 respectively, for average delay for the BRT and BRT + VMS scenarios). The increased BRT (exclusive of ROW) benefit that resulted from considering reliability measures can be attributed mainly to the following factors: 1. Including reliability in the route choice for automobile and transit resulted in added impedance (penalty) for both modes. Because automobile travel was less reliable than the BRT option with exclusive ROW in the study corridor, the inclusion of reliability naturally increased the ridership of BRT. 2. The subsequent increased ridership of BRT allowed a higher number of travelers in the corridor using a more reliable BRT mode. Thus, when computing the person- based MoEs for the entire corridor, the results arguably captured the benefit of BRT more accurately than a case with no reliability measure inclusion. 3. The incremental benefit of VMSs was more pronounced when reliability was considered but not the other way around. In other words, when reliability was incorporated the ability of VMSs to properly balance the traffic load between I-5 and Barbur Boulevard in time of congestion could be captured by the model. Figure 7.9. Southbound I-5 corridor measure of effectiveness (MoE) comparisonâwithout reliability. Figure 7.10. Southbound I-5 corridor measure of effectiveness (MoE) comparisonâwith reliability.
