Subproblem 5b - Page 6 of 7
Traffic Signal Control with a Protected Westbound Left Turn
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
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.
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