Survey Methodology
The National Occupant Protection Use Survey (NOPUS) is the only probability-based
observational survey of safety belt use in the United States. The survey
observes usage as it actually occurs at a random selection of roadway sites,
and so provides the best tracking of the extent to which motorists in this
country are buckling up.
Sites, Vehicles, and Motorists Observed
| Numbers of |
2003 |
2004 |
Percentage Increase |
| Sites Observed |
2000 |
2000 |
0% |
| Vehicles Observed |
162,000 |
146,000 |
-10% |
| Occupants Observed1 |
214,000 |
193,000 |
-10% |
| 1 Drivers and right front passengers only. |
The survey data is collected by sending trained observers to probabilistically
sampled roadways, who observe vehicles between the hours of 8 AM
and 6 PM. Observations are made either while standing at the roadside
or, in the case of expressways, while riding in a vehicle in traffic.
Observers do not stop vehicles or interview occupants, so that
the NOPUS captures the untainted behavior of motorists. The 2004
NOPUS data were collected between June 7 and July 11, 2004, excluding
the period July 2 – 5 inclusive, while the 2003 data were
collected between June 2 and July 1, 2003.
Because the NOPUS sites were chosen through probabilistic means, we can analyze
the statistical signi-ficance of its results. Statistically significant increases
in belt use between 2003 and 2004 are identified in the table “Safety Belt
Use by Major Characteristics” by having a result that is 90% or greater
in the table’s column 7. Significantly high and low levels of belt use,
such as the lower use in rural areas than in more populated areas in 2004, are
identified by H’s and L’s in columns 3 and 5.
The NOPUS uses a complex multi-stage probability sample, statistical data editing,
imputation of unknown values, and complex estimation and variance estimation
procedures. See the NHTSA Technical Report referenced at the end of this note
for more information on these procedures.
Data collection, estimation, and variance estimation for the NOPUS are conducted
by Westat, Inc. under the direction of the National Center for Statistics and
Analysis in NHTSA under federal contract number DTNH22-00-D-07001.
Definitions
A State, D.C., or Puerto Rico, has a “primary enforcement law” if
motorists can be ticketed simply for not using their belts. Under a “secondary
enforcement law” motorists
must be stopped for another violation, such as an expired license tag, before
being cited for belt nonuse. In June 2003, 18 States had primary laws, 32 had
secondary laws, and 1 State (New Hampshire) effectively has no belt law. (In
New Hampshire, it is legal for motorists over age 18 to ride unbelted.) Primary
enforcement laws took effect in Delaware and Illinois in July 2003, and in Tennessee
in July 2004.
The “conversion rate” is the percentage reduction in belt nonuse.
This rate roughly reflects the percentage of belt nonusers in 2003 who were “converted” to
using belts in 2004.
“Expressways” are defined to be roadways with limited access, while “surface
streets” comprise all other roadways.
A roadway is defined to have “fast traffic” if during the observation
period the average speed of passenger vehicles that passed the observer(s) exceeded
50 mph,
with “medium speed traffic” defined as 31 - 50 mph and “slow
traffic” defined as 30 mph or slower.
A roadway is defined to have “heavy traffic” if the average number
of vehicles per lane mile on the roadway during the observation period exceeded
45 vehicles
per lane mile, with “moderately dense traffic” defined as 26 - 45
vehicles per lane per mile and “light traffic” having at most 25
vehicles per lane per mile.
For More Information
For detailed analyses of the data in this publication, as well as additional
data and information on the survey design and analysis procedures, see the upcoming
publication “Safety Belt Use in 2004 – Overall Analysis”, expected
to be available at the web site http://www-nrd.nhtsa.dot.gov/departments/nrd-30/ncsa/AvailInf.html in
October 2004.
The NOPUS also observes other types of restraints, such as child restraints and
motorcycle helmets, and observes driver cell phone use. This publication is part
of a series that presents overall results from the survey on these topics. Please
see other members of the series, such as “Motorcycle Helmet Use in 2004 – Overall
Results”, and the corresponding NHTSA Technical Report “Motorcycle
Helmet Use in 2004 – Analysis”, for the latest data on these topics.
