Sub-problem 4b: Clifton Country Road PM peak hour - Existing Conditions

Lost Time
Start-up lost time is the seconds of green that go unutilized because it takes the lead vehicles a little while to get going. The HCM default is 2 seconds. Sometimes, the intersection geometry lengthens this figure. In this case, the northbound and southbound approaches have a slightly longer lost time, because of an upgrade and the geometry of the left-turn moves. It takes the discharging queues a short while to get organized before vehicles start to flow smoothly. Increasing the start-up lost time to 3 seconds, which is what we’ve assumed, lets us account for the delay due to the slope.

At the end of the green, you also have to specify the extension of effective green. This is the number of seconds, after the light goes yellow, that vehicles are still entering the intersection. The HCM assumes 2 seconds. The HCM's default assumptions, then, are that the specified green time is the same as the length of the effective green time. The ideas are different, but the numbers are the same. If you have a green 20 seconds long, and a yellow that’s 3 seconds long, then the lost start-up time means that vehicles are moving during only 18 of the 20 seconds of green, losing 2 seconds. On the other hand, if you assume a green time extension of 2 seconds, vehicles are still entering the intersection for 2 of the 3 seconds of yellow, gaining the two seconds of lost time.

By assuming the start-up lost time is 3 seconds and leaving the extension of green unchanged at 2 seconds, we assume the amount of effective green available to the northbound and southbound flows is one second shorter than the green time we assign. If the green time is 20 seconds, subtracting 3 and adding back 2 means we have 19 seconds of effective green.