A study of rural roadside-interview sampling techniques.

An evaluation was made of four roadside-interview sampling techniques, based on complete bidirectional origin- destination data collected for one 7 day week in each of June, July and September at a rural location in southern Ontario. The four sampling techniques tested were unidirectional interviewing, bidirectional interviewing (alternate half-hours and alternate hours in each direction) and proportional bidirectional interviewing (interviewers assigned to each direction in proportion to the directional traffic volumes). Each method tested assumed the same number of interviewers. In all cases, unidirectional interviewing produced poorer trip estimates than bidirectional interviewing. A first series of tests on same day traffic compared individual daily factored, sampled o-d trip movements for each interviewing method with the actual o-d trip movements for the same day. The bidirectional method produced trip estimates that were comparable in accuracy and significantly better than the trip estimates produced by unidirectional interviewing. Another series of tests on average weekday (and) traffic compared individual, daily, factored, sampled o-d trip movements for each interviewing method with the actual o-d trip movements averaged over the 5 weekdays. The bidirectional methods again produced trip estimates comparable in accuracy, but the errors were only slightly smaller than those produced by unidirectional interviewing. The reduced error advantage for bidirectional interviewing in estimating awd trip movements was caused by the daily variation error: that is the variability of o-d trip movements from day to day within the week. Close agreement was found between actual and theoretical awd or 'combined' errors (which incorporates the effects of both sampling and daily variation errors). Proportional bidirectional interviewing is recommended because of its accuracy and its flexibility and safety characteristics. Several tests on the stability of total and interdistrict trip purpose volumes at the survey location are also reported.