were also more likely to be heavy drinkers according to the QFV index, and to have
used a DD in the last 12 months. Although these variables were consistent with the
univariate results, the overall model was not good at predicting which respondents
had used a SR when a 0.50 classification level was used. Of the 29 SR users with
non-missing data in the equation, none were classified correctly by the model. This
was primarily a result of the low frequency of SR users. The model classified
correctly all but one non-user and had an overall classification rate of 92 percent.
Due to the low frequency of SR users, a different classification level is more
appropriate, therefore, a receiver operating characteristic function was computed to
determine the classification level that maximized the sensitivity and specificity of the
model. A classification criteria of 0.136 maximized the predictive value of the model
such that 75 percent of SR users, 83 percent of non-users, and 82 percent of subjects
overall were classified correctly.
Frequency of SR Use
In examining data for barroom survey respondents who report using SRs (N = 42),
the use of SRs was shown to be relatively infrequent (3.69 times per person in the last
12 months). Over 95 percent of respondents reported six or fewer rides, with 69 percent
reporting only 1 or 2. Since one individual reported receiving 57 rides (approximately
one ride per week), excluding this one case drops the group mean to 2.39 rides per
person. Clearly, most respondents reported using the service infrequently.
In Situ Breath Tests
The ride-along assessments of a separate sample of 51 SR users also suggests that
SR users tend to be higher risk and heavier drinkers. The majority of SR users were
male (67%), 47 percent of users were alone, and 98 percent of the SRs were provided
from public drinking establishments as opposed to private homes. The drinking drivers'
BACs, as measured by breath test, ranged from 0.031 to 0.248, with an average of
0.146. Seventy-nine percent of SR users tested had BACs above 0.10, the legal limit
for DWI in the State of Maryland.