Street Design Manual

Where traffic is to be controlled by a traffic signal, the turn lane should be of sufficient length to store the turning vehicles and clear the equivalent lane volume of all other traffic on the approach, whichever is the longest. This length is necessary to ensure that full use of the separate turn lane will be achieved and that the queue of the other vehicles on the approach will not block vehicles from the turn lane. The storage requirements for turn lanes should be based upon the peak 15-min. flow rates of turning traffic. The average number of turns per cycle can then be multiplied by a factor to account for random variations in arrivals.

The length of turn lane can be estimated by the following formula: = ∗ ∗ ∗ ( + ) Where, L = storage length of turn lane (feet) V = peak 15 minute flow rate of turning volume (vph) K = constant to reflect random arrivals

K = 2.0 for 95% probability of storing all vehicles K = 1.5 for 90% probability of storing all vehicles 25 = approximate length of vehicle (feet per vehicle) N = number of cycles for hour

Raleigh Street Design Manual – Page 92