Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.0Tij=Aj∗P ∗ fi∑ Pi′ ∗i′∈Zones( dij)f ( d )ij′whereT ij is the predicted flow produced by Zone i and attracted to Zone jP i is the predicted number of trips produced by Zone i from trip generationA j is the predicted number of trips attracted by Zone j from trip generationd ij is the impedance (distance, travel time/cost) between Zone i and Zone jf(d ij ) is the friction factor between Zone i and Zone j.The truck model employs a doubly constrained approach that alternatively balances the productions byapplying the first equation and then balances the attractions by applying the second equation. Thiscontinues until a convergence criterion is met.The role of the friction factor is simply to adjust the relative attractiveness of a potential destination zoneas a function of its distance, travel time or cost from the origin zone. Observed truck trip lengths werenot available for this study; therefore, it was not possible to derive a table of friction factors or tocalibrate impedance function parameters.In the absence of such data, the QRFM (USDOT 1996, p. 4-19) recommends an exponential friction factorfunction:f− ∗tij( t ij) = eαwhere:t ij is the travel time (skim) between Zone i and Zone jα is an impedance parameter.The values of α recommended in the QRFM are 0.10 for single-unit trucks and 0.03 for multi-unit trucks.Mathematically, these parameters imply a mean trip length of 10 minutes for SU trucks and 33 minutesfor MU trucks. The chart in Figure 2-4, below, depicts these relationships graphically. The relationship isone in which destination zones become less attractive as one travels further from the origin. This curveis steeper for SU trucks, which are used primarily for local hauls, and less steep for MU trucks, which aremore often used for inter-city and other longer distance hauls. Use of these friction factors implies thatSU truck trips will be primarily local in range, with few expected to go beyond 60 minutes in length. Incontrast, MU trucks will carry the bulk of inter-city freight, with trips beyond two-hours in length fairlycommon.The problem with using this exponential function is that, in reality, very short distance truck trips areoften less attractive because of the costs of loading and operating a truck. This is particularly true of thelarger MU trucks and to a lesser extent for SU trucks. MU trucks include tractor-trailer combinations:standard box shape trailers, flatbed trailers, large tankers and the two-tiered trailers used to haulautomobiles. Even in heavily industrialized zones, freight movements within the zone would be moreefficiently carried out with smaller single-unit (SU) trucks. If trip-length distribution data were available,it would likely result in a curve that takes on a lognormal shape, or some similar functional form thatproduces low-value friction factors close to the origin, rising sharply as travel time increases, peaking anddecreasing gradually over longer distances.Truck Model - Base-Year Truck Model Development 17