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

Right Turns on Red
In all of the datasets for the Clifton Country Road intersection we assumed there are a substantial right turns on red (RTOR) for the eastbound, westbound, and northbound approaches. (click here to open any one of the datasets.) Each of these three approaches has an auxiliary right turn lane. The one difference among them is that the eastbound right-turn lane is quite short and the right turn on red is often difficult to make because the lane is blocked, the westbound lefts are turning, or the southbound throughs are in motion.

Exhibit 2-44 shows you the difference in predicted performance between the base case PM Existing condition and condition where the RTOR on the eastbound, westbound and southbound approaches are prohibited.  Follow the link to see Dataset 38 (2002 PM base case) or Dataset 39 (RTOR prohibited) datasets. In the base case, 10% of the eastbound rights turn right on red. For the westbound approach, all of the right turns are omitted because there is a separate right-turn auxiliary lane and there isn’t much opposing traffic. For the northbound approach, as many vehicles turn right during phases 1 and 2 as westbound lefts in a single lane (550/2). Another 10% of the northbound rights move during other times when the light is red northbound. Southbound, there are no right turns on red. In the RTOR prohibited dataset, all the right-turn-on-red volumes are set to zero except for the northbound approach where the volume is set to 275, reflecting the number of right turns that move when the westbound left is moving.

Notice that the eastbound and westbound RTOR values can be zero and the intersection can still function well. Neither of these RTOR prohibitions causes a catastrophic increase in delay. The eastbound right-turn delay grows from 14.0 to 14.5 seconds per vehicle. The westbound delay grows from 13.1 to 16.3 seconds per vehicle. The base case assumes that these auxiliary lanes are reachable, meaning that the length of the eastbound lane must be 10-20 cars long to be reachable 50-95% of the time, and the westbound lane must be 10-19 cars long.

For the northbound approach, a RTOR prohibition creates LOS F (these results are not shown in the table.)  In fact, the smallest feasible RTOR value is 297 vehicles. That is, if the RTOR value was decreased from the base case, through some traffic engineering treatment, the smallest it could be would be 297 vph and still have LOS F for the northbound rights.