Table of Contents
- Demand-Based Site Selection for Fire Stations Using a Network Allocation Model
- Setting up Your Workspace
- Preparing the Data
- Skill Drill: Clip the Roads Layer to the City of Arcata and McKinleyville
- Skill Drill: Add a Time Cost Attribute to the Roads Layer
- Skill Drill: Creating a CSV Table to Import As XY Data
- Skill Drill: Geocoding an Address and Creating a CSV Table to Import As XY Data
- Skill Drill: Adding the Fire Incident History as XY Data
- Creating A Network Dataset
- Setting up a Network Allocation Model
- Loading Facilities into the Model
- Loading the Demand Points into the Model
- Adjusting the Analysis Setting of the Model
- Running the Network Allocation Model
- Skill Drill: Using the Maximize Attendance Problem Type
- Skill Drill: Adding Existing Fire Stations to the Model
- Skill Drill: Creating a Map of the Results
Skill Drill: Add a Time Cost Attribute to the Roads Layer
To calculate the travel time, in minutes, you will use the length of each road segment along with the speed limit. The road layer should have the length of each segment recorded in the Shape_Length field. However, since your have clipped the road layer, these values will need to be updated to account for any road segments that may have been altered during the Clip operation. To update the length values, use the Calculate Geometry option in the attribute table. If you will recall from previous courses, you can access Calculate Geometry by right clicking on the field name, Shape_Length. Make sure the values are saved in meters.
Note: If you can’t find the Shape_Length field, you can create your own by adding a new field in the attribute table. Then use Calculate Geometry to populate the field.
The network allocation model requires a cost attribute with time units to be present in the network dataset. Create a new field in the attribute table of the clipped-roads layer. Name this field Minutes. This field will record the travel time, in minutes, for each road segment. In most cases, the road segments will have values of less than one minute travel time. Therefore, the field type should be either a float or a double. If you are ever unsure which, I recommend that you always choose the double field type since it will allow for a larger number if necessary.
The road layer has two attributes that record the speed limit, AB_Speed, and BA_Speed. These attributes represent the speed limit, in miles per hour, for each direction. For consistency, use the AB_Speed in your calculation. Use the Field Calculator to compute the travel time and to populate the Minutes field. Your formula should factor in the AB_Speed (miles per hour) and the Shape_Length (meters) of each road segment to determine the travel time in minutes. If you will recall from previous courses, you can access Field Calculator by right-clicking on the field name, Minutes.
Coming up with the right formula will involve converting miles per hour to meters per minute. Then it requires using the length of the roads to determine the travel time in minutes.
Once you have successfully calculated the minutes, you can verify the accuracy of your calculation by checking the travel time of a portion of G Street in Arcata, just before the on-ramp to HWY 101. Just west of the on-ramp, the travel time of the road segment on G Street is 1.059253 minutes. See the image below to compare the same road segment with your own data.