Capacity study for protected-permitted phasing from shared lanes

Protected-permitted phasing from shared-use lanes (i.e., no left-turn bays) exists in certain situations where space and other constraints make this measure expedient in the face of growing traffic demand. An alterative method for handling this special case is proposed as an enhancement to the existing treatment in the 1997 update to the Highway Capacity Manual. The method proposed is logical and consistent. The more practical issue of whether it is also more accurate is addressed in a series of simulation cases. Simulation was used to provide the reference conditions because of field costs, number of variables to be controlled, and the fact that a well-established model exists. The results show that the existing method systematically overestimates lane group capacity by 10 to 25 percent for this special case, whereas the proposed alternative estimates the lane group capacity by ±10 percent or less.