Lec 11 TD-Part 4: 5.4.3 & 5.5.1, H/O pp.491: Mode Usage (modal split) and Intro to trip assignment Lecture Objectives • Understand why modal split needs to be done (5.4.3) • Understand the difference between Direct-generation usage models (Trip-end models) and Trip-interchange mode usage models (H/O, pp. 491 -) • Understand the meaning of the logit formulation (5.5.1) • Learn how to use these models (H/O, pp. 491 -)
What is modal split? • Split trips to different available transportation modes, by analyzing people’s decisions regarding mode of travel they use. Can be done here Trip-end models vs. Trip-interchange models
What affects people’s mode choice? Characteristics of the trip: trip distance, time of day, trip purpose Characteristics of the trip maker: Income, # of autos available, family size, residential density, gender Characteristics of the transportation system: riding time, waiting time, transfers, out-of-pocket cost
Direct-generation usage models (Trip-end models) Generate trips for transit and highway users separately meaning transit users use only transit (“captive” users). Used for small communities or in developing countries where ridership is primarily a function of socioeconomic variables
Direct-generation usage models (Trip-end models) (cont) Same categories but different trip rates Or, use separate models, like: P(T) = A + B(POP) – C(INC) P(A) = A + B (POP) – C(AUTO) Advantage: Simplicity Disadvantage: Cannot reflect “change of mind” of trip makers responding to policy and service changes
Trip-interchange models Influencing all three phases. Trip-interchange models are used AFTER the trip distribution phase.
Trip-interchange models (cont) Competing modes Because trip-interchange models are used after trip distribution, they can utilize the service characteristics of the modes available for the given trip, along with any relevant socioeconomic characteristics to determine the modal splits. This is the preferred and overwhelmingly typical approach for urban areas in which significant transit service exists and in which the “competition” between auto, transit, and other modes of travel must be explicitly considered.
Trip-interchange models (cont) Let’s see how service and trip makers characteristics can affect the trip maker’s decision using Fig. 11-11. • In-vehicle time (Auto – Transit) = -15 min • Out of pocket cost (Auto – Transit) = 25 cents • Excess time (Auto – Transit) = 3 min • If so, 37% of trips will be made by transit. Looks like a logit curve…
Use of logit models for modal choice (“Disaggregate, random utility modal choice model”) The logit model trades off the relative utilities of various modes. “The better a mode is, the more utility it has for the potential user” (See Examples 11& 12)