Sub-problem 5b: Three-phase Traffic Signal Control with a Protected Westbound Left Turn

These observations reinforce the notion that, in the search for an equitable distribution of green times, there is a tradeoff between equalizing the v/c ratios and equalizing the delays. The question of which strategy is preferable raises an interesting philosophical question. Note that equalizing the delays has reduced the overall intersection LOS from C to D. So a reporting scheme that considers only the overall LOS would tend to favor the equal v/c strategy. On the other hand, the improvement in overall intersection LOS was achieved at the expense of the lower volume movements that were forced to operate at LOS E and F. So, a reporting scheme that is concerned with individual movements might look more favorably on equalizing the delay.

This debate might spawn a third strategy, namely that of equalizing the LOS among the competing movements without worrying too much about differences in delay. The results would be expected to fall somewhere between the two strategies that we have explored.

Now here is a question to ponder: why is the overall intersection delay of 42 sec per vehicle lower than the delays for any of the movements shown in Exhibit 3-39? The answer is that our analysis has focused on the critical movements and has neglected other movements such as the WB through traffic, which was not involved in any of the computations for the signal timing strategies we explored. The procedure prescribed by the HCM for estimating overall intersection LOS takes all movements into account, not just the critical movements.