13.2.2 Study Limits
The study limits for a project are determined during the scoping process. The study limits incorporate the project limits plus the area of influence, which have an impact on the operations in the study area. Wherever applicable, it is recommended that the model extends a minimum of one intersection or interchange both upstream and downstream of the farthest intersection or interchange within the study area. below shows an
example
from the FHWA Interstate System Access Informational Guide of potential study limits on an interstate project
Figure 13-1: Area of Influence
Source: FHWA’s Interstate System Access Informational Guide
See
Section 13.6
of this chapter for more information about incorporating unmet demand into a microsimulation model. Preferably, travel speeds at the study limits remain near free-flow speeds throughout the analysis period. This allows the modeler to fully capture the formation and dissipation of congestion within the study area limits.13.2.2.1 Freeways
It is recommended that models extend at least one interchange (including ramp terminals) beyond the study limit in both the upstream and downstream directions. This additional area is included during the calibration and debugging process, but results are not always reported. All intersections with significant influence on the arrival patterns of vehicles entering the network are typically included in the model. If extending the study limit is not practical, additional information on capturing unmet demand is provided in
Section 13.6
.13.2.2.2 Arterials
Arterial model networks typically capture both one signalized intersection upstream and one signalized intersection downstream of the study intersections. If signalized intersections are not present, such as in rural areas, it is recommended that the network capture approximately half a mile of the arterial in both the upstream and downstream directions. Visual inspection of field data is used to determine if more of the roadway network will be included in the analysis. Like freeways, all intersections with significant influence on the arrival patterns of vehicles entering the network are included in the model. Additionally, it is recommended that all resulting congestion during the peak period be captured within the study limits. If congestion is identified on a network boundary, extend the study limits to capture the entirety of the queue. If extending the study limits is no longer practical, additional information on capturing unmet demand is provided in
Section 13.6
.