Once the trip generation rates had been initially determined, the trips generated by residential areas were distributed (outbound) to the retail and employment areas (inbound) based on the ratio of shopping and job trips received by each retail and employment zone. Employment zones included jobs outside of Lacombe; these trips were assigned to the four external nodes on the boundaries of Lacombe.
Trips generated by the existing retail and employment zones areas were distributed (outbound) to the residential zones (inbound) based on the percent population in each transportation zone. The in and outbound trips were then balanced against each other and compared to the raw trip generation rates assumed for each zone. Screen line volumes were also assessed at various locations within the Town to verify the volumes passing particular intersections and links.
In this manner (and by comparing the output with the observed traffic volumes throughout the Town) it was possible to determine (a) where too much or too little traffic had been assigned to/from a particular zone or zones, (b) routes that had been assigned too much or too little traffic based on existing traffic distributions, and (c) areas where the trip generation rates may have been too high or too low.
This process of calibration was repeated several
times for the existing conditions until the forecast distribution and traffic
volumes were within 20% of the observed traffic volumes on the roadways. At this
point, it was confirmed that the model was calibrated and that the trips
generation rates, trip routes, and distribution to and from the various zones
was accurate to a point that would be suitable for forecasting purposes. The
distribution used in the development of the 20,000 population horizon traffic
forecasts is summarized here in Table
8.5.