Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.0The FAZ system was overlaid upon this truck network and new FAZ centroid nodes and connector linkswere created. The larger size of the FAZs meant that, in many cases, highways or other roads passedthrough a zone rather than paralleling its border as in the consolidated TAZ system. This was not viewedas a problem, since the purpose of the network was to aid in the creation of truck trip tables that willeventually be assigned to the same full network and consolidated TAZ system as the passenger vehicles.A map of the final truck network is shown in Figure 2-3, below. External stations are represented bysquares. FAZ centroids and centroid connectors are not shown.2.1.3 Vehicle classificationMuch of the literature on commercial vehicle freight movement classifies trucks into three broadcategories: four-tire trucks (e.g., pick-up trucks, sport-utility vehicles and vans); single-unit trucks (e.g.,bread and other local delivery trucks, garbage trucks, package delivery trucks such as UPS and FedEx);and articulated/combination trucks (e.g., tractor semi-trailers, tankers, flatbed trailers). These are usefulgroupings for modeling purposes because trucks within these categories tend to have similar travelpatterns.It was decided that the truck model would focus on the single-unit and combination (hereafter multi-unit)truck classifications. Single-unit (SU) and multi-unit (MU) trucks can be identified with reasonableaccuracy by automatic traffic recorders, based on the number of axles and distance between them.Generally, SU trucks have six or more tires and are thus differentiated from smaller commercial vehicles,so-called light trucks such as pickups, vans and mini-vans. In terms of behavioral characteristics, SUtruck trips are generated at greater rates than MU truck trips; however, MU trucks tend to havesubstantially greater average trip lengths because they dominate the long-haul trucking market.Four-tire trucks were excluded from the truck model because, unlike SU and MU trucks, they are rarelyused for hauling freight, are nearly impossible to validate from traffic count information, and,theoretically, should be accounted for in the home interview survey as non-home-based work trips.During the model development process, some concern was expressed over the decision to excludecommercial four-tire trucks from the truck model. The principal concern was that many non-home basedtrips, especially for work, go unreported in travel diaries. The following explanations were offered for thisdecision:• The percentage of four-tire trucks used primarily for freight transportation is not known, but shouldnot be large. Most four-tire trucks used for freight are likely to operate only short distances andthus would not be expected to constitute a large share of the trucks operating in the NSTI studyarea.• To include four-tire trucks in the truck model could lead to substantial double counting with thepassenger-vehicle model, particularly for non-home-based trips. The majority of four-tire trucks areregistered as personal vehicles. According to the 1997 Vehicle Inventory and Use Survey (U.S.Bureau of the Census), 73 percent of all trucks registered in Ohio were for personal use (68 percentin Kentucky and 71 percent in Indiana). The remainder include commercial, exempt (primarilyagriculture), and government registrations. The conventional wisdom, however, is that most fourtiretrucks are used for transporting persons or for service trade. A plumber, for example, might usegoods carried on his truck in the performance of a service call, but transporting these items is notthe primary purpose of the trip. This is an important distinction because these trips are alreadyincluded in the person-vehicle model. Moreover, many commercial vehicle drivers, particularly insmall businesses, use these vehicles for both work and personal travel.Truck Model - Base-Year Truck Model Development 8