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2019 VERSION: Volume III: Guidelines for Applying Traffic Microsimulation Modeling Software 2019 Update to the 2004 Version

2004 Version - Appendix F: Demand Constraints

Microsimulation results are highly sensitive to the amount by which the demand exceeds the capacity of the facility, so it is vital that realistic demand forecasts be used in the analysis. The following steps outline a procedure for manually reducing the forecasted demands in the study area to better match the capacity of the facilities feeding the study area.

Step 1: Identify Gateway Bottlenecks

The analyst should first identify the critical bottlenecks on the facilities feeding the traffic to the boundaries of the microsimulation study area. Bottlenecks are sections of the facilities feeding the model study area that either have capacities less than other sections of the freeway or demands greater than the other sections. These are the locations that will probably be the first ones to experience congested conditions as traffic grows.

These bottlenecks may be located on the boundary of the microsimulation study area, in which case they are identical to the gateway zones on the boundary of the microsimulation model study area. Bottlenecks within the microsimulation model study area can be disregarded since they will be taken into account by the microsimulation model.

Inbound bottlenecks are congested sections feeding traffic to the microsimulation model area. Outbound bottlenecks are congested sections affecting traffic leaving the microsimulation model area. If an outbound bottleneck will probably create future queues that will back up into the microsimulation model study area, then the model study area should be extended outward to include the outbound bottleneck. If the future outbound queues will not back up into the model study area, then these bottlenecks can be safely disregarded (see Figure 19).

Figure 19. Bottleneck, gateway, and study area. Diagram. This figure contains a shaded oval area, Microsimulation Model Study Area, which is crossed by three lines. A bottleneck is drawn outside the shaded oval area on one of the lines, and directional arrows indicate movement both away from and toward the shaded oval area. The word Gateway is at the outside border of this area, along the same line as the bottleneck. Directional areas at the gateway indicate movement both into and out of the area.

Figure 19. Bottleneck, gateway, and study area.

Step 2: Estimate Excess Demand at Inbound Bottlenecks

If the forecasted hourly demand at a bottleneck (in the inbound direction toward the model) exceeds its capacity, the proportion of the demand that is in excess of the available hourly capacity should be computed:

Equation 30. The proportion of excess demand, X, equals the forecasted demand (vehicles per hour), D, minus estimated capacity (vehicles per hour), C, all divided by C.  (Equation 30)

where:

X = proportion of excess demand

D = forecasted demand (veh/h)

C = estimated capacity (veh/h)

Step 3: Reduce Forecasted Demand Inbound at Gateways

The forecasted hourly demands for the off-ramps between the bottleneck and the gateway entering the microsimulation study area should be reduced in proportion to the amount by which the forecasted bottleneck demand exceeds its capacity:

Equation 31. The constrained demand (vehicles per hour) for a downstream off-ramp or exit point, D subscript const, equals the unconstrained demand forecast (vehicles per hour), D subscript unconst, times the sum of 1 minus X.  (Equation 31)

where:

Dconst = constrained demand (veh/h) for a downstream off-ramp or exit point

Dunconst = unconstrained demand forecast (veh/h)

X = proportion of excess demand

It is suggested that the off-ramp demand be reduced in proportion to the reduction in demand that can get through the bottleneck, assuming that the amount of reduction in the downstream flows is proportional to the reduction in demand at the bottleneck. If the analyst has superior information (such as an O-D table), then the assumption of proportionality can be overridden by the superior information. The constrained downstream gateway demand is then obtained by summing the constrained bottleneck, off-ramp, and on-ramp volumes between the bottleneck and the gateway to the study area.

Figure 20 illustrates how the proportional reduction procedure would be applied for a single inbound bottleneck that reduces the peak-hour demand that can get through from 5000 veh/h to 4000 veh/h. Since there is an interchange between the bottleneck and the entry gate to the microsimulation study area, the actual reduction is somewhat less (800 veh/h) at the gate.

Figure 20. Example proportional reduction of demand for capacity constraint. Diagram. This figure is a magnified view of Figure 19. It contains the lower portion of the shaded oval area, Microsimulation Model Area, with the bottleneck in the lower right corner outside the area. Directional arrows indicate movement toward the area. Before the bottleneck, D subscript UC equals 5000, and D subscript C equals 5000. Immediately after the bottleneck, D subscript UC equals 5000, and D subscript C equals 4000. Exiting at an off-ramp, D subscript UC equals 1000, and D subscript C equals 800. Entering at the on-ramp, D subscript UC equals 500, and D subscript C equals 500. Entering the microsimulation model area, D subscript UC equals 4500, and D subscript C equals 3700.

Figure 20. Example proportional reduction of demand for capacity constraint.

Starting upstream of the bottleneck, there is an unconstrained demand for 5000 veh/h. Since the bottleneck has a capacity of 4000 veh/h, the downstream capacity constrained demand is reduced from the unconstrained level of 5000 veh/h to 4000 veh/h. Thus, 1000 vehicles are stored at the bottleneck during the peak hour. Since it is assumed that the stored vehicles are intended for downstream destinations in proportion to the exiting volumes at each off-ramp and freeway mainline, the downstream volumes are reduced the same percentage as the percentage reduction at the bottleneck (20 percent). A 20-percent reduction of the off-ramp volume results in a constrained demand of 800 veh/h. The on-ramp volume is unaffected by the upstream bottleneck, so its unconstrained demand is unchanged at 500 veh/h. The demand that enters the microsimulation study area is equal to the constrained demand of 4000 veh/h leaving the bottleneck, minus the 800 veh/h leaving the freeway on the off-ramp, plus 500 veh/h entering the freeway at the on-ramp, which results in a constrained demand of 3700 veh/h.

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