When two or more upstream lanes are connected to a single downstream lane, a merging area is defined (see Figure 3.4). In the merging area, additional constraints are considered in calculating acceleration rate because vehicles from adjacent upstream lanes may need to coordinate with each other. For the purposes of the simulator, merging is classified into: (i) priority merging; and (ii) non-priority merging. Priority merging includes merging from ramps to freeways, and from minor streets to major streets. Non-priority merging occurs at the downstream of toll plazas, and lane drops on freeways.
For priority merging, a vehicle without right-of-way must check whether there is any vehicle from the competing upstream lanes. The vehicle executes the merge only if the projected headway gap is acceptable. If the headway gap is not acceptable, the vehicle either calculates the car-following acceleration rate by treating the vehicle which has the right-of-way as leader or it prepares to stop at the end of the lane (depending on which case is the critical one).
For non-priority merging, a ``first come, first serve'' principle determines right-of-way. In other words, among all the vehicles coming from competing upstream lanes, the one closest to the downstream lane is chosen as the first vehicle to merge. Other vehicles will either follow appropriate leaders or prepare to stop at the end of their current lanes.
A merging area consists of an upstream area (e.g. 100 feet) and a downstream (140 feet) area. The downstream area is also characterized by the maximum number of vehicles allowed in. An upstream merging area vehicle (i.e. vehicles D or E) is tagged as a merging vehicle if it has yet to be tagged and the number of downstream merging area vehicles have not reached the maximum. All vehicles in the downstream merging area (i.e. vehicles B and C) are tagged as merging vehicles. The acceleration rates for merging vehicles is calculated by relaxing the car-following constraint (negative gap from the immediate leading vehicle is allowed) and incorporating a merging constraint (no overtaking is allowed).
Vehicles are untagged when they leave the merging area, and the merging constraints are then relaxed.
