Traffic Safety Facts

Traffic Tech – Technology Transfer Series

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

National Highway Traffic Safety Administration

Number 383 March 2010

Guidelines to Observe and Estimate Statewide 

Seat Belt Use at Night

Previous research has found an overrepresentation of fatal 

crashes at night, as well as substantially lower seat belt use 

among occupants of passenger vehicles at night compared 

to seat belt use during the day. An estimate of statewide 

nighttime seat belt use would be beneficial for understanding the problem of nighttime driving risk, and for evaluating 

the impact of nighttime seat belt enforcement programs. The 

Guidelines to Observe and Estimate Statewide Seat Belt Use at 

Night provides States with options for estimating statewide 

seat belt use at night. This report also provides a detailed 

description of how to conduct night observations.

Background

The National Highway Traffic Safety Administration standardized the way States measure seat belt use by introducing 

probability-based procedures and guidelines for selecting 

observation locations and weighting data collected at those 

sites. The weighting served to make the observations representative of statewide seatbelt use based on both traffic 

volume and population. The sampling and weighting with 

regard to traffic volume primarily used 24-hour traffic information. The fact that volume information is based on all the 

hours of the day, though observations were done only in 

daylight, was not an issue given that the large majority of 

traffic occurs during the daytime hours. The estimates generated by these surveys provide a good measure of belt use 

during the day. 

Some recent studies have attempted to provide an estimate 

of statewide nighttime seat belt use. Generally, however, 

they failed to properly account for certain aspects of changes 

in traffic from night to day. There are fewer motor vehicles on 

the roadways at night as compared to the day. However, the 

drop-off in volume is not consistent between different functional classes of roadways. Higher volume roadways (e.g., 

interstates and freeways) tend to retain more of their volume 

at night than lower volume roadways (e.g., local roads and 

collectors). The problem with applying the daytime weighting to the nighttime observation data is that the distribution 

of the traffic across functional class at night is unlikely to be 

the same as it is during the day. Therefore, the weighting will 

overrepresent the influence of some functional classes and 

underestimate the influence of others. 

The guidelines in this report are based on taking the existing design for estimating (daytime) belt use and adjusting 

the weighting to accommodate differences in nighttime traffic volumes. For States having comprehensive hour-by-hour 

traffic volumes, there are also guidelines to draw an entirely 

new sample of observation sites, based on nighttime traffic 

data, to use in estimating nighttime seat belt use. 

Re-Sampling Based on Night Data

States may opt to draw an entirely new sample on which to 

conduct night observations. This new sample would be independent from the day survey. States should consider drawing a new sample of road segments for their nighttime survey 

only if the appropriate data are available. Specifically, a State 

would need to have a measure of hourly traffic volume by 

vehicle type for all roadways eligible to be sampled. If such 

data are available, a State may opt to then follow the same 

procedures used in drawing the sample for the daytime survey, but base selection probabilities on the nighttime passenger vehicle volumes (i.e., exclude buses and heavy trucks) 

rather than the 24-hour traffic volumes used for daytime 

sample selection. Details are available for the guidelines from 

NHTSA. For convenience and comparability the nighttime 

sample would follow the daytime design in terms of how 

many road segments, of what types, and in what counties. 

Re-Weighting Daytime Plan for Nighttime 

Observations

Most States will not have the data necessary to draw a completely new sample for night observations. Therefore, they 

will need to start with their existing daytime sample. The 

formulas that are used to combine all data into a statewide 

daytime belt use figure will need to be adjusted for nighttime 

belt use computation. The adjustment is simple: replace the 

24-hour weights with new weights based on nighttime passenger vehicle traffic. The report describes how to reweight 

the existing daytime weighting spreadsheet to estimate statewide nighttime belt use from nighttime observation data.

Data weighting methods differ greatly from State to State. 

The report provides general guidelines for weighting night-
time observation data to produce an unbiased estimate with 

minimal variance. The main issues are:

n change in proportion of traffic occurring on individual 

functional classes from day to night, and

n overrepresentation of large truck traffic at night compared 

to day. 

The new weighting will start from the existing weighting 

spreadsheet for a State’s daytime survey. The traffic volume data used to make the daytime data representative of 

statewide (daytime) belt use is inappropriate for weighting nighttime observation data and producing an estimate 

of statewide nighttime seat belt use. Vehicle miles traveled 

(VMT) data will need to be adjusted to make the weights 

representative of night traffic in terms of passenger vehicle 

distribution across functional classes. This should be the only 

necessary adjustment, since population, the other variable 

typically used in weighting, is constant across time of day. 

The adjustments can be made in one of two ways depending 

on what data are available to the State. If the State can access 

hourly traffic volume data (i.e., vehicle counts from each 

road segment for each hour of the day) then the weighting 

will be performed slightly differently than if the State only 

has access to total 24-hour (i.e., daily) volume for each segment. Note that the segments described here are not just the 

observation sites but the complete sample of all roadways in 

the State used to calculate State VMT. 

Both methods will require “clicker counts,” that is, actual 

counts of traffic volume, to estimate the volume of passenger 

vehicles at night. The hourly data will likely result in more 

reliable estimates of night traffic volume, and therefore some 

effort should be invested into obtaining this data if available.

The report describes the steps for reweighting under each 

situation (hourly versus daily data availability) and provides 

examples from two States, Pennsylvania and Connecticut. The 

general purpose of the steps is the same for all States, but the 

steps may need to be adjusted to achieve their goals depending on the weighting scheme for any individual State. 

Summary

The general steps used to perform a statewide estimate of 

nighttime seat belt use are to (1) keep everything the same 

as the daytime estimate except for the few things required to 

make it possible to conduct the observations, and (2) make 

the observations representative of nighttime traffic. 

Some enhanced procedures need to be implemented during nighttime observations primarily because it is usually 

dark and difficult to see at night. Judicious adjusting of site 

locations, special equipment, observer/recorder teams, and 

enhanced safety measures will overcome most of the obstacles posed for seat belt observations at night.

Strategies to make the observations representative of night 

traffic include drawing an entirely new sample based on 

night traffic volume data, and making the volume data 

already included in the standard weighting spreadsheet represent night passenger vehicle volume. 

How to Order

To order Guidelines to Observe and Estimate Statewide Seat 

Belt Use at Night (32 pages), prepared by Preusser Research 

Group, write to the Office of Behavioral Safety Research, 

NHTSA, NTI-130, 1200 New Jersey Avenue SE., Washington, 

DC 20590, fax 202-366-7394, or download from www.nhtsa.

gov. Angela Eichelberger, Ph.D., was the project manager.

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 Dr. Angela

Eichelberger, Editor, by sending a fax to 202-366-7394, or e-mail 

angela.eichelberger@dot.gov. 

1200 New Jersey Avenue SE., NTI-130

Washington, DC 20590