Number 325 May 2007

Traffic Safety Facts

Traffic Tech – Technology Transfer Series

A Study of Nighttime Seat Belt Use in Indiana

U.S. Department of Transportation 1200 New Jersey Ave. SE., Washington, DC 20590

National Highway Traffic Safety Administration

Background

The United States has made great strides in increasing use

of seat belts, with nationwide belt use now exceeding 80

percent according to NHTSA’s National Occupant Protection Use Survey (NOPUS). The NOPUS, however, as well

as nearly all other direct observation studies of seat belt

use, are conducted during daylight hours. Recentresearch

suggests that nighttime seat belt use may be lower than

belt use rates during the day. To date, those studies have

been conducted in a limited number of geographic areas.

The objective of this project was to obtain a Statewide

measure of nighttime belt use in a geographic area where

no such Statewide measure had as yet been obtained, and

compare that measure to the State’s daytime usage. A secondary goal was to assess change in daytime and nighttime usage associated with the May 2006 Click It or Ticket

(May Mobilization) campaign.

Methods

Based on multiple criteria, Indiana was chosen as the State

in which to conduct the nighttime seat belt observation

survey. Two nighttime surveys were planned by this project, one immediately preceding the May 2006 Mobilization and one at its conclusion. The nighttime surveys were

designed to match as closely as possible daytime belt observation surveys being conducted by the State of Indiana

both before and after the Mobilization. The daytime surveys were Statewide probability-based surveys meeting

all of the NHTSA requirements under section 157. The

nighttime surveys used the same sites, and made every

attempt to collect data from each site during the same

night of the week as the daytime data was collected. A

longer nighttime field period necessitated by the expense 

of the night vision equipment precluded same-day day

and night data collection for all sites.

Trained field staff observed shoulder belt use, sex, age, 

seating position, vehicle type, and vehicle purpose (commercial or noncommercial) of drivers and front-right

passengers during nighttime hours. Motorists traveling

in passenger cars, sport utility vehicles, large vans, minivans, pickup trucks, and box trucks were included in the

data collection. Data collection for the first nighttime wave 

(pre-Mobilization) took place April 17-30, 2006; data collection for the second nighttime wave (post-Mobilization)

occurred June 5-18, 2006.

Nighttime Seat belt Use

The overall nighttime seat belt use results are presented

in Table 1. The nighttime statewide belt use rates for Indiana were 79.0 ± 1.9 and 74.0 ± 2.2 percent, respectively

for waves 1 and 2. Z-tests were performed to compare

means across survey waves. The results of these analyses

allowed for rejection of the null hypothesis at the p<0.001

level. In other words, overall nighttime seat belt use decreased at a statistically significant level across the mobilization period. Table 1 also shows seat belt use rates by

vehicle seating position. The z-tests performed on the belt

use rates within each seating position across the survey

waves revealed that belt use significantly decreased for 

drivers (p<0.001), but not for passengers. During wave 1,

belt use was statistically significantly higher for passengers than drivers (p<0.05), with no significant difference 

observed during wave 2.

As is typically found in seat belt use research, belt use

for males was significantly lower than that for females in 

both survey waves. The z-tests that were performed to test

these means found the difference to be statistically significant at the p<0.001 level for each wave. Statistically significant decreases were also observed within each sex across

the survey waves (p<0.05: males; p<0.01: females).

Nighttime seatbeltuse ratesbyvehicle type are also shown

in Table 1. When comparing the belt use rates within each

vehicle type across the survey waves, statistically significant decreases in seat belt use were observed for pickup

truck and SUV occupants (p<0.05).

As is typically found in seat beltresearch, motorists traveling in pickup trucks had the lowest observed seat belt use

rates of any vehicle type during both survey waves. These
TRAFFIC TECH is a publication to disseminate information about traffic safety programs, including evaluations,

innovative programs, and new publications. Feel free to

copy it as you wish. If you would like to receive a copy, contact Melissa Cheung, MPH, Editor, fax 202-366-7096, e-mail:

traffic.tech@dot.gov. 1200 New Jersey Ave. SE., NTI-130

Washington, DC 20590

rates were significantly lower (p<0.001) than the rates observed for occupants of cars, minivans, and SUVs. Belt

use for those in pickup trucks was also significantly lower 

than for large van occupants during wave 2 (p<0.05), but

not during wave 1. These results are also not surprising

since the seat belt use law for Indiana exempts motorists

traveling in pickup trucks. The z-tests performed on the

belt use rates for motorists traveling in large vans, revealed that these occupants displayed significantly lower 

belt use than those traveling in cars and minivans during both survey waves (p<0.05). During wave 1, this rate

was also statistically lower than the observed belt use of

SUV occupants (p<0.05). When rates for occupants of cars,

minivans, and SUVs were compared with one another,

the only significant difference was observed during wave 

2 between those in cars and SUVs (p<0.05).

Table 1: Nighttime Shoulder Belt Use, Confidence Interval, 

and Unweighted n by Wave, Vehicle Type, and Subgroup

Wave 1: Pre-Mobilization Wave 2: Post-Mobilization

Percent 

Use ± CI n Percent Use ± CI n

Overall 79.0 ± 1.9 3,896 74.0 ± 2.2 5,003

Seating Position

Driver 78.6 ± 2.0 3,139 73.4 ± 2.3 3,937

Passenger 82.0 ± 2.9 757 76.7 ± 5.0 1,066

Sex

Male 73.1 ± 3.1 2,345 68.2 ± 2.6 2,951

Female 88.2 ± 2.0 1,538 83.2 ± 2.7 2,047

Vehicle Type

Car 83.1 ± 2.4 2,303 81.5 ± 2.8 2,917

SUV 83.0 ± 3.2 619 76.5 ± 5.1 791

Minivan 82.9 ± 4.4 352 84.8 ± 5.3 464

Large Van 67.8 ± 16.9 50 66.8 ± 13.5 66

Pickup 61.7 ± 6.2 572 52.2 ± 6.0 765

Comparison of Daytime to Nighttime

Seat Belt Use

The nighttime survey waves were designed, conducted,

and analyzed by the NHTSA contractor. The two daytime waves were designed and conducted by contractors

(Center for the Advancement of Transportation Safety,

or CATS) working for the State of Indiana. Overall daytime rates reported here are based upon data generously

provided to the NHTSA contractor by CATS to allow

for study comparisons. These data were analyzed by the

NHTSA contractor for comparison purposes using the

same procedures utilized by CATS.

Overall belt use rates observed during the daytime and

nighttime survey waves are presented in Table 2. Belt use

observed during the daytime and nighttime pre-mobilization waves was very similar. During the post-mobilization

waves, daytime belt use was 10.3 percentage points higherthan nighttime use. The two-tailed z-tests performed on

these data revealed a significant difference at the p<0.001

level. Comparison of the two daytime waves to one another also revealed a statistically significant increase in 

daytime belt use following the mobilization activities

(p<0.001). Additionally, as mentioned earlier, the overall

analysis of the pre- and post-mobilization nighttime survey waves revealed a statistically significant decrease in 

seat belt use during this time period.

Table 2: Overall Daytime and Nighttime Seat belt Use 

Within Each Wave (Percent Use, Confidence Interval, 

Unweighted n)

Daytime Pre Daytime Post

Percent Use ± CI n Percent Use ± CI n

79.7 ± 1.3 19,077 84.3 ± 1.4 19,934

Nighttime Pre Nighttime Post

Percent Use ± CI n Percent Use ± CI n

79.0 ± 1.9 3,896 74.0 ± 2.2 5,003

How to Order

To order A Study of Nighttime Seat Belt Use in Indiana 

(29 pages), prepared by the University of Michigan

Transportation Research Institute (UMTRI), write to the

Office of Behavioral Safety Research, NHTSA, NTI‐130, 

400 Seventh Street SW., Washington, DC 20590, fax 202-

366-7096, or download from www.nhtsa.dot.gov. Alan

Block was the Contracting Officer’s Technical Representative for this project.