Journal of Accident Investigation

KRISTIN M. POLAND, LINDA McCRAY, AND AIDA BARSAN-ANELLI
the direct impact zone had a high probability of surviving the
crash with only minor to moderate injuries.
Sled Testing
Sled tests were conducted to replicate the acceleration
time history of the school bus full-scale frontal impact test.
The derived sled pulse 13 agreed very well with the time
duration (approximately 210 msec) and the peak acceleration
(approximately 12-13 g). The leveling off of the acceleration
pulse of the crash test from about 40 to 90 msec was a result
of the bus body sliding along the chassis. The sled test did not
exactly replicate this plateau and allowed a somewhat higher
acceleration level at this point in the curve. The result was
a slightly more severe test pulse because the peak velocity
of the sled was approximately 4 to mph higher than the
barrier-equivalent velocity measured during the frontal crash
test.
Two different test bucks were used to evaluate bus safety
restraint systems. For the first test buck, a section from the
body of the school bus was mounted on the sled. This test buck
was used to assess the degree of deformation/energy absorption
by the bus floor and the floor’s interaction with the seats, and
to assess any potential for occupant interaction with portions
of the interior other than the seats themselves. The finished
test buck is shown in figure 1 . The bus body contained three
rows of seats on both the right and left side of the center aisle.
This allowed for testing a maximum of two rows of dummies
per test.
Figure 15. This photograph shows the sled buck for
the frontal crash testing.
Testing with the first sled buck showed no significant
interaction between the dummies and the walls or ceiling of
13 “Report to Congress on School Bus Safety: Crashworthiness Research,
April 2002” page 19, Figure 6, at http://www-nrd.nhtsa.dot.gov/
departments/nrd-11/SchoolBus/SBReportFINAL.pdf .
the bus shell, although the floor of the bus shell sustained some
incremental damage from loading by the mounted bus seats. 14
For that reason, an open-frame, rigid-floor, test buck was used
to provide a more consistent test platform. (See figure 16.)
This test buck allowed for better high-speed imaging of the test
event, which improved the analyses of the dummies and their
interaction with the bus seats and restraint systems.
Figure 16. A photograph of the modified sled buck used
in the second test series.
The sled tests were designed to evaluate occupant size,
restraint strategies, loading conditions, seat spacing, and seat
back height. The occupants were represented by Hybrid III
0th-percentile male dummies, Hybrid III th percentile female
dummies, and Hybrid III 6-year-old dummies. The restraint
strategies that were evaluated included compartmentalization,
use of lap belts (with compartmentalization), and use of lap/
shoulder belts on bus seats with a modified, non-FMVSS
222-compliant seat back. 1 Other conditions evaluated were
seat spacing, seat-back height, and rear occupant loading.
Results indicated that compartmentalization is an effective
restraint strategy for frontal school bus crashes, in part due to
the relatively low acceleration load in even a relatively severe
crash condition. The padded seat backs appear to be effective
in minimizing the potential for leg and head injury. During the
frontal crash, occupant kinematics were such that chest loading
was not a significant problem. However, these conditions
created some degree of risk for neck injury as measured by the
14 The deformation was very small for any single test and accounted for an
insignificant amount of energy absorbed by the seats during the crash
simulation.
1 Code of Federal Regulations, Title 49, Chapter V–National Highway Traffic
Safety Administration, Department of Transportation, Federal Motor
Vehicle Safety Standard No. 71.222; School Bus Passenger Seating and
Crash Protection.
64 NTSB JOURNAL OF ACCIDENT INVESTIGATION, SPRING 2006; VOLUME 2, ISSUE 1