# Applying Topological Information for Routing Commercial Vehicles Around Traffic Congestion
## 4 Methodology and Model Development
### Diversion Model
#### 4.4.4 Heavy Vehicle Restrictions  
The National Bridge Inventory data were inspected to identify criteria that could be used as restrictions on the network. Bridges with insufficient design loads (less than H1) were added to the network in a new layer as restrictions to prevent the diversion of truck traffic to these locations. Bridges with vertical clearance less than 14 feet were also added to the network as restrictions.  
Bridge inventory data was extracted as a CSV file and converted to a shapefile, and the layer was projected and plotted to ArcGIS. The bridge points were clipped to the study area. Bridges with a design load less than H1 or with low vertical clearance were selected, and a new layer with the selection was created. This feature class was buffered by 0.5 miles. These buffers were added as scaled-cost polygon barriers in Network Analyst. Figure 7 presents the selection criteria that were considered in this study.  
Figure 7: Alternative route selection criteria.  
The developed model finds all potential alternative routes throughout the study area, taking into consideration heavy vehicle restrictions and the impacts of traffic diversion on neighborhood and traffic conditions. Predefined alternative route selection criteria were incorporated in the network dataset. Based on the criteria for lane width, capacity, and vertical clearance, those routes with features that made them unacceptable as alternate routes were excluded from consideration. A penalty factor (scaled cost) was also applied to roads within a defined distance of schools or hospitals.  
Alternative route selection criteria were added to the network as restrictions or scaling factors to assign penalties to each segment that intersected a specified criterion. Additionally, estimated travel time, restrictions, and barriers such as low-clearance bridges and school zones were considered.  
The algorithm was designed to find the shortest path, excluding scalable costs and constraints; this would be the base scenario. Another scenario was also considered that added cost factors and restriction barriers. This scenario simulates the truck diversion route. The Network Analyst extension produces turn-by-turn instructions for each simulation scenario to define alternative routes to avoid high-risk interstate closures.  
Neighborhood selection criteria impacts consider the properties of nearby land use along the proposed truck path. Truck intrusions into residential areas and near other critical facilities, such as schools or parks, are not desirable. Truck restrictions can be introduced to prevent truck drivers from driving on roads near these land uses.  
Also, road conditions selection criteria consider both the level of traffic congestion along the proposed road route and its capacity. Roads carrying traffic volumes approaching capacity are less attractive as potential truck routes, as traffic congestion would have a detrimental effect on freight movements.  
Case studies were developed to evaluate the potential alternative route and validate the proposed model. These scenarios were compared with the base scenario, which is travel on the main route during incident conditions.