Figure 5-6 shows the percentage of miles during which there was nonuse of the seat belt during each
phase of the experiment. The data are shown separately for each seat position. For all four seat
positions, the failure to use a seat belt was lowest during some portion of the treatment period;
nonuse was generally higher during the baseline and transfer phases. Since seat belt usage was quite
high for the front seat positions (about 95% during baseline), the improvement in seat belt usage
during the treatment phases was not large in absolute terms, but remained consistent. Given that the
majority of teen vehicle occupant fatalities were unbelted, even a relatively small increase in the
percentage of belted occupants has meaningful safety implications. However, as indicated in Table
4-3, the main effect of treatment stage was statistically significant only for the right side rear seat
position. Although this relatively small-scale evaluation did not have the power to discriminate small
shifts in the rate of seat belt use, this should not be taken to imply that there is no benefit to the
system, given the potential safety benefits of even small shifts in usage rates.
0.00%
0.20%
0.40%
0.60%
0.80%
1.00%
1.20%
Baseline Immediate Short Term Long Term Transfer
3-4 m/s² (>0.3-0.4 g's)
4-5 m/s² (>0.4-0.5 g's)
5+ m/s² (>0.5 g's)
49
Analyses conducted on the SBDRS data indicated a significant main effect for time of day for the
right side rear passenger position. The data indicated a reduction in seat belt compliance of 15.7
percent for the right side rear passengers at night when compared to daytime conditions. The
reduction in compliance for this specific position between day and night is not well understood and
may reflect the increased risk taking of rear seat passengers at night when driving with fellow teens.
The follow-up paired comparisons for the main effect of stage for the right side rear passenger
indicated an increase of 16.5 percent in the compliance rate between the immediate and short-term
treatment stages (see Figure 5-6). These results are particularly promising as it shows rear passengers
are influenced by the feedback and that teens may have been enforcing seat belt compliance with the
assistance of the system. As an example, teens noted that the STC feedback helped drivers identify
the lack of seat belt compliance by passengers that the STC gave drivers an excuse to enforce
compliance (e.g., the system is telling you to buckle up). It is important to note that caution should
be taken when interpreting the data from the rear passenger area. The amount of rear passenger data
collected in this study was quite limited compared to front seat passengers. The rear passenger data
were less consistent and less abundant than the data collected from the driver or the front passenger.
While the results are encouraging, future research should identify and further monitor rear seat belt
compliance.
The observations were supported by anecdotal comments from teens who responded that the
SBDRS provided a clear message about seat belt use and also helped them establish a habit of seat
belt checks for all seating positions.
A significant issue relative to this subsystem was reliability. A few participants noted that seat belt
buckle magnets had become dislodged that in turn would give off warnings even though the person
was buckled. In a few instances false alarms occurred where the system issued a warning for one of
the seating positions where nobody was seated. Both teens and parents suggested a greater need to
increase the sensitivity for identifying actual passengers so that trust in the system would be
maintained.
Recommendations: The SBDRS reminds and motivates teen drivers and occupants to use their seat
belts. It is recommended the SBDRS be retained in future STC configurations to maintain high seat
belt compliance rates for drivers and front passengers and to promote seat belt compliance among
rear passengers. It is recommended that the multistage reminder system be retained. In an effort to
encourage seat belt compliance, the system activates in two specific stages. The first stage gives the
teen a mild reminder where the teen is prompted to buckle their seat belt and the second stage only
occurs if there is continued non-compliance. It is also recommended that the subsystem be coupled
with the passenger presence subsystem so that redundant seating information can be exchanged
between the subsystems and thus increase the accuracy and reliability of the SBDRS.