Transportation Planning: Demand Forecasting
The four-step model
Trip generation estimates productions and attractions using ITE-style rates (per dwelling unit, per 1,000 sf, per employee) or regression equations; trip distribution allocates them with a gravity or deterrence model; mode choice converts person trips to vehicle trips using logit/utility models; assignment loads vehicles onto the network, often with a BPR volume-delay function. Model validation compares estimated versus counted volumes and screenlines.
Planning context
Know functional classification (arterial, collector, local), the MPO's role in the metropolitan planning process (long-range plan plus TIP), concurrency requirements, and performance measures such as VMT, person-miles traveled, and mode share.
Traps
- Confusing the peak hour of the generator with the peak hour of the adjacent street - they often differ by an hour or more.
- Forgetting to subtract internal capture (mixed-use), pass-by trips (retail, typically 20–40%), and diverted linked trips before calling the result "new" trips.
- Treating person trips as vehicle trips without applying vehicle occupancy.
- Using an average rate for a very large generator when the published equation is more appropriate.
Worked example
A 150,000 sf retail center generates 40 vehicle trips per 1,000 sf per day: 150 × 40 = 6,000 daily trips. If 30% are pass-by, new external trips are 6,000 × 0.70 = 4,200. In a mixed-use development, an additional internal capture would be removed before the pass-by adjustment, since internal trips never touch the external street.