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2 CHAPTER 2 THE METHODOLOGY The NCHRP 25-21 methodology is designed to answer one and how it is satisfied on the transportation system. This effect fundamental question, "Will a specified traffic-flow improve- is very long term. (Extensive transportation capacity invest- ment contribute to improved or worsened air quality locally ments in one metropolitan area can also increase the net in- USER'S GUIDE and at the regional level, in the short term and in the long migration to the region, but this effect will not be considered term?" Repeated exercise of the methodology on various case in this research.) studies will answer the question, "Under what conditions will Thus, the NCHRP 25-21 methodology addresses four basic a specified traffic-flow improvement contribute to improved or mechanisms by which traffic-flow improvements can influ- worsened air quality?" ence mobile source emissions: Operational improvements that smooth out traffic flow 2.1 THEORETICAL FOUNDATION and thus reduce acceleration/deceleration events, Travel time savings and losses on particular routes and The NCHRP 25-21 methodology proceeds from the funda- modes of travel that influence the traveler's choice of the mental theoretical foundation that "nobody travels for the fun of it." People travel in order to participate in activities or to most cost-effective means for satisfying their demand to obtain goods that are superior to what they could have done travel, Travel time savings that increase the total demand for or obtained at their original location. Even sightseers use the transportation to experience a vista they could not see at travel, and Travel time savings that increase the relative attractive- home. They may say they enjoy the drive, but what they really enjoy is what they can see out of the window. The research ness and therefore the growth rate of subareas in the team will exclude from this blanket statement individuals region. who test their vehicles or are hired to drive a vehicle. Travel demand is, therefore, not vehicle-miles traveled The first mechanism, operational improvements, will be (VMT). Travel demand is the schedule of activities, by loca- called the "operations" effect. Traffic flow improvements tion, that travelers would like to pursue that day. In modeling may increase the average speed on the facility, and/or they parlance, it is the origin-destination (OD) table of person may increase the capacity of the facility prior to affecting trips for that day by time of day. However, VMT is the most travel behavior. Operational improvements will also affect cost-effective measure (from the traveler's point of view) for vehicle mode of operation activity by reducing acceleration measuring that demand. and deceleration events. The operations effect occurs on the Thus, traffic-flow improvements by reducing average travel first day that an improvement is opened for traffic. Travel- times can affect both the total demand for travel and the trav- ers have not yet had an opportunity to change their demand eler's choice of the most cost-effective means for satisfying schedule in response to the travel time savings provided by that demand. the improvement. In addition, some traffic-flow improvements do not change The second and third effects of traffic-flow improvements the average travel time but reduce the variance in travel speeds will be combined into a single "traveler behavior" effect. This by smoothing out the traffic flow. Thus, it is possible for a effect comes in the months following opening day. As travel- traffic-flow improvement to have no effect on demand or on ers become aware of the improvements, they change route, how that demand is satisfied on the street system and yet still mode of travel, and departure time to take advantage of them. have an effect on air quality by smoothing out the "stop-and- After the improvement has been in place for sufficient time go" nature of the trip itself. for travelers to change their demand schedule (e.g., the OD Finally, a series of traffic-flow improvements can make one table), they will take advantage of the reduced travel costs portion of the metropolitan area more attractive to growth and brought about by the improvement. Traveler behavior effects new development than older, more established parts of the include changes in destination choice and trip generation region. The shifting of growth from centrally located devel- (extra trips or stops along the way of a preexisting trip). The oped areas to undeveloped fringe areas can affect both demand result of the behavior effects will be to partially counteract the