Capacity Estimation for Two-Lane Two-Way Highways Using Simulation

The Highway Capacity Manual 2000 (HCM 2000) indicates that the capacity of two-lane, two-way highways (TLTWHW) is 1,700 pcph for a single direction and 3,400 pcph for both directions. There are no capacity estimates provided for different traffic conditions such as varying percentages of no-passing zones and varying traffic distributions between the two directions of travel. Previous research has reported that capacity conditions for TLTWHW are very difficult to observe in the field, while none of the existing simulation models can report capacity for TLTWHW. This paper briefly describes a new two-lane, two-way highway simulation model, called TWOSIM that is developed, verified, compared to TWOPAS and HCS, and used for the estimation of capacity for TLTWHW under a variety of prevailing traffic and geometric conditions. These include percentage of traffic per direction, presence of a passing zone, a horizontal curve, a driveway, an upgrade, and heavy vehicles. Capacity of TLTWHW under base conditions is found to range from 1,800 pcph to 2,100 pcph as a function of average free flow speed. The presence of passing zones was not found to have an effect on capacity. When a driveway or horizontal curve or upgrade is present, the capacity reduction ranges between 3 and 30 percent when there are no trucks, and may reach up to 40 percent when trucks are present in the traffic stream.