PEAK SPREADING MODELS: PROMISES AND LIMITATIONS

This paper discusses a new time-of-day departure time choice model, or "peak spreading" model, as developed for the San Francisco Bay Area. The model is a simple binomial logit choice model with the choices of AM peak (two-hour) period departure and non-AM peak period departure. The choice is applied to daily home-to-work auto person trips. This home-based work departure time model is estimated using data from the 1990 Bay Area household travel survey, using data variables such as free-flow and AM peak period congested travel time, trip distance, household income, and dummy variables for bridge crossers, carpooling and retail employment. Highway assignments were calibrated and validated against 1990 daily and peak period traffic volumes and peak period speeds. The problems with using this mode in future year forecasts are discussed. This simple peak-spreading model has a tendency to divert trips from the peak period to the shoulders of the peak period due to increased congestion levels. The result is that the peak period traffic volumes are sometimes lower than the peak shoulder period traffic volumes, yielding too fast speeds in the peak period and too slow speeds in the shoulder periods. This is our "snow plow" effect, with traffic piling up on the shoulders to allow traffic to flow during the peak period. The quick fix to this problem was to prepare four-hour AM peak period traffic assignment based on peaking factors derived from household travel surveys. The slower of the two-hour and four-hour AM peak period assignments are used to feed back to all mode choice models for purposes of forecast equilibration. Other logit departure time models have been estimated and are discussed. They include multinomial choice models (peak period, shoulder period, off-peak period) and binomial choice model (peak period, shoulder period) based on four-hour AM peak auto person trips.