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225
<br />impact ®f ®ensity and Transit ®n driving
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<br />300 600
<br />900
<br />Zonal Transit Density
<br />mortgage. Location Efficiency analysis is already being extended to allow LEMs in Seattle,
<br />Washington and Portland, Oregon. In neither of these are mileage readings for vehicles available.
<br />The dominance of Veh/Iili in the calculations of VMT/Hh facilitated these extensions. Veh/ffii,
<br />available from the U.S. Census in all cities, along with densities, income, household size, transit
<br />service and pedestrian and bicycle friendliness, allow the Veh/Iih equation to be calibrated.
<br />Average VMT/Veh is calculated by state departments of transportation based upon fuel use,
<br />screenline monitoring and other gross measures. This allows the VMTNeh equation to be
<br />calibrated to the regional average VMT/Veh.
<br />Extending the analysis to other important dependent variables.-vehicle trips and vehicle starts -
<br />would require localized measures of these. The closest available measures are household travel
<br />surveys, but as with VMT analysis these contain insufficient data. For instance, in the San
<br />Francisco region there average less than ten households per zone. The scarce data problem is
<br />aggravated by the lack of any households in many zones, especially the denser zones, which are
<br />scarcer to begin with. Since fewer trips commence in denser zones, and are consequently less
<br />critical to transportation planning, planners have tended to slight them. But aggregating zones by
<br />residential density provides more respectable household numbers, allowing derivation of a gross
<br />relationship of trips to VMT by residential density. This relationship could be used to extend the
<br />predictions of VMT/Hh to trips/household. Similarly, the time between trips, if reported in the
<br />household travel survey, could be used to extend the predictions to cold starts.
<br />Similarly, the impacts on trips and VMT of job and commerce concentrations could be estimated
<br />by use of household travel surveys. Jobs and local shopping employees are known by zone.
<br />Grouping the zones by local shopping density or job ranges would allow shopping trips
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