TRAFFIC SAFETY FACTS
Crash • Stats

DOT HS 812 506

A Brief Statistical Summary

March 2018

Critical Reasons for Crashes Investigated in theNational Motor Vehicle Crash Causation Survey
Summary
The National Motor Vehicle Crash Causation Survey(NMVCCS), conducted from 2005 to 2007, was aimed at collecting on-scene information about the events and associated factors leading up to crashes involving light vehicles.
Several facets of crash occurrence were investigated during
data collection, namely the pre-crash movement, critical precrash event, critical reason for the critical pre-crash event, andthe associated factors. In each investigated crash, these wereassigned to driver, vehicle, or environment without suggesting that any of these was cause of the crash or assignmentof fault to driver, vehicle, or other crash element. A sample
of 5,470 NMVCCS-qualified crashes (i.e., the crashes meetingcertain criteria listed in the section: Scope and Limitationsof NMVCCS data) was investigated over a period of two anda half years, which represents an estimated 2,189,000 crashes
nationwide. About 4,031,000 vehicles, 3,945,000 drivers, and1,982,000 passengers were estimated to have been involvedin these crashes. The critical reason, which is the last eventin the crash causal chain, was assigned to the driver in 94
percent (±2.2%)† of the crashes. In about 2 percent (±0.7%)of the crashes, the critical reason was assigned to a vehicle
component’s failure or degradation, and in 2 percent (±1.3%)of crashes, it was attributed to the environment (slick roads,weather, etc.). Among an estimated 2,046,000 drivers whowere assigned critical reasons, recognition errors accountedfor about 41 percent (±2.1%), decision errors 33 percent (±3.7%),
and performance errors 11 percent (±2.7%) of the crashes.

Introduction
Databases such as the National Automotive Sampling
System (NASS) Crashworthiness Data System (CDS) do notprovide information on pre-crash scenarios and the reasons
underlying the critical pre-crash events. In 2005, the National
Highway Traffic Safety Administration (NHTSA) was authorized under Section 2003(c) of the Safe, Accountable, Flexible,Efficient, Transportation Equity Act: A Legacy for Users(SAFETEA-LU) to conduct a national survey to collect onscene data pertaining to events and associated factors thatpossibly contributed to crash occurrence. NHTSA’s National

Center for Statistics and Analysis (NCSA) conducted theNMVCCS from July 3, 2005, to December 31, 2007. Crashes
were investigated at the crash scene to collect driver, vehicle,
and environment-related information pertaining to crashoccurrence, with a focus on driver’s role. The targeted information was captured mainly through four data elements: (i)movement prior to critical pre-crash event (i.e., the movementof the vehicle immediately before the occurrence of the critical event); (ii) critical pre-crash event (i.e., the circumstancethat led to vehicle’s first impact); (iii) critical reason for thecritical pre-crash event (i.e., the immediate reason for the critical event, which is often the last failure in the causal chainof events leading up to the crash); and (iv) the crash associated factors (i.e., the factors that are likely to add to the probability of crash occurrence). This was done with reference tothe crash envelope that comprises of a sequence of events,
referring to the above data elements, which eventually led
to the crash. Refer to the section Scope and Limitations ofNMVCCS data.
This Crash•Stats presents some statistics related to one of theabove-mentioned four data elements, namely “critical reason
for the critical pre-crash event.” The data obtained throughthe sample of 5,470 NMVCCS crashes and the weights associated with them were used to obtain national estimates offrequencies and percentages along with their 95-percent confidence limits, as presented in the following sections.

Critical Reasons for the Critical Pre‑Crash Event
The critical reason is the immediate reason for the critical pre-crash
event and is often the last failure in the causal chain of events leading up to the crash. Although the critical reason is an importantpart of the description of events leading up to the crash, it is notintended to be interpreted as the cause of the crash nor as the assignment of the fault to the driver, vehicle, or environment.
A critical reason can be assigned to a driver, vehicle, or environment. Normally, one critical reason was assigned percrash, based upon NMVCCS researcher’s crash assessment
without suggesting the cause of the crash or assignment offault. The critical reason was assigned to the driver in an esti-

95% conf. limits: ± tα/2; deg. freedom × Std. Dev. (α = 0.05, t-value = 2.179)†Published by NHTSA’s National Center for Statistics and Analysis		

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2

mated 94 percent (±2.2%) of the NMVCCS-qualified crashes(Table 1). In addition, the critical reason was assigned to thevehicle in an estimated 2 percent (±0.7%) and to the environment in about 2 percent (±1.3%) of the crashes.

Table 2. Driver-Related Critical Reasons

Table 1. Driver, Vehicle, and Environment Related CriticalReasons

Recognition Error

845,000

41% ±2.2%

Decision Error

684,000

33% ±3.7%

Performance Error

210,000

11% ±2.7%

Non-Performance Error (sleep, etc.)

145,000

7% ±1.0%

Other162,000

8% ±1.9%

Total

2,046,000

100%

NumberEstimated
Percentage*± 95% conf. limits

Drivers

2,046,000

94% ±2.2%

Vehicles

44,0002% ±0.7%

Environment

52,0002% ±1.3%

Unknown Critical Reasons

47,0002% ±1.4%

2,189,000

100%

Critical Reason
Attributed toTotal*Percentages are based on unrounded estimated frequencies(Data Source: NMVCCS 2005–2007)

The critical reasons related statistics are presented in detail in
Table 2 for drivers, Table 3 for vehicles, and Table 4 for environment.

Critical reason attributed to drivers
The critical reason was assigned to drivers in an estimated
2,046,000 crashes that comprise 94 percent of the NMVCCS
crashes at the national level. However, in none of thesecrashes was the assignment intended to blame the driverfor causing the crash. The driver-related critical reasons arebroadly classified into recognition errors, decision errors,performance errors, and non-performance errors. Statistics
in Table 2 show that the recognition error, which included
driver’s inattention, internal and external distractions, andinadequate surveillance, was the most frequently assigned(41% ±2.2%) critical reason. Decision errors such as driving
too fast for conditions, too fast for the curve, false assumption of others’ actions, illegal maneuver and misjudgment of
gap or others’ speed accounted for about 33 percent (±3.7%)of the crashes. In about 11 percent (±2.7%) of the crashes,
the critical reason was performance error such as overcompensation, poor directional control, etc. Sleep was the mostcommon critical reason among non-performance errors that
accounted for 7 percent (±1.0%) of the crashes. Other drivererrors were recorded as critical reasons for about 8 percent(±1.9%) of the drivers.

Critical Reason

Estimated (Based on 94% of theNMVCCS crashes)
Percentage*
Number
± 95% conf. limits*Percentages are based on unrounded estimated frequencies(Data Source: NMVCCS 2005–2007)

Critical reason attributed to vehicles
The critical reason was assigned to vehicles in an estimated
44,000 crashes comprising about 2 percent of the NMVCCS
crashes, though none of these reasons implied a vehicle
causing the crash. There were no detailed inspections ofvehicles during the NMVCCS on-scene crash investigation;
the vehicle-related critical reasons were mainly inferredthrough external visual inspection of the vehicle components. This resulted in only mostly external, easily visible
factors (tires, brakes, steering column, etc.) that were citedas the few vehicle-related critical reasons. The related statistics may not, therefore, be representative of the role of otherinternal vehicle related problems that might have led to thecrash. Of the small percentage (2%) of the crashes in which
the critical reason was assigned to the vehicle, the tire problem accounted for about 35 percent (±11.4%) of the crashes.
Brake-related problems as critical reasons accounted forabout 22 percent (±15.4%) of such crashes. Steering/suspension/transmission/engine-related problems were assigned
as critical reasons in 3 percent (±3.3%) of such crashes. Other
vehicle-related problems were assigned as critical reason inabout 40 percent (±24.0%) percent of such crashes.

Table 3. Vehicle-Related Critical Reasons

Critical Reason

Estimated (Based on 2% ofthe NMVCCS crashes)
Percentage*
Number ± 95% conf. limits

Tires /wheels-related

15,00035% ± 11.4%

Brakes-related

10,00022% ± 15.4%

Steering/suspension/transmission/engine-related

2,000

3% ± 3.3%

Other/unknown vehicle-related problems

17,00040% ± 24.0%

Total44,000100%*Percentages are based on unrounded estimated frequencies(Data Source: NMVCCS 2005–2007)

Published by NHTSA’s National Center for Statistics and Analysis		

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3Critical reason attributed to environment

Scope and Limitations of NMVCS Data

The critical reason was assigned to about 2 percent of theestimated 2,189,000 NMVCCS crashes. However, none ofthese is suggestive of the cause of the crash. Table 4 presents statistics related to crashes in which the critical reason
was attributed to roadway and atmospheric conditions. Inabout 50 percent (±14.5%) of the 52,000 crashes the criticalreason was attributed to slick roads. Glare as a critical reason
accounted for about 17 percent (±16.7%) of the environmentrelated crashes, and view obstruction was assigned in 11 percent (±7.2%) of the crashes. Signs and signals accounted for 3
percent (±2.5%) of such crashes. In addition, in 52,000 of thecrashes with a critical reason attributed to the environment,
the weather condition (fog/rain/snow) was cited in 4 percent(±2.9%) of the crashes.

NMVCCS data is restricted to crashes that occurred from
6 a.m. to midnight. In addition, the following criteria must
be met by a crash to qualify for an on-scene investigation(NMVCCS-qualified crash):n The crash must have resulted in a harmful event associated with a vehicle in transport on a trafficway.

n EMS must have been dispatched to the crash scene.
n At least one of the first three crash-involved vehicles
must be present at the crash scene when the NMVCCS
researcher arrives.

n The police must be present at the scene of the crash when
the NMVCCS researcher arrives.

Table 4. Environment-Related Critical Reasons
Estimated (Based on 2% of theNMVCCS crashes)
Percentage*
Number± 95% conf. limitsn At least one of the first three vehicles involved in theSlick roads (ice, loose debris, etc.)

26,00050% ±14.5%

n A completed police accident report for this crash must be

Glare

9,000

17% ±16.7%

View obstructions

6,000

11% ±7.2%

Other highway-related condition

5,000

9% (0, 9.9)††%

Fog/rain/snow

2,000

4% ±2.9%

Other weather-related condition

2,000

4% (0.0, 9.1)††%

Signs/signals

1,000

3% ± 2.5%

Road design

1,0001% (0, 3.3)††%

Total

52,000100%

Critical Reason*Percentages are based on unrounded estimated frequencies††Conf. limits with lower limit 0(Data Source: NMVCCS 2005–2007)

crash must be a light passenger vehicle that was towed
or will be towed due to damage.
available.
It is important to note that the pre-crash assessment of crashes
in terms of the critical event, critical reasons, and associatedfactors is neither suggestive of the cause of the crash nor ofa fault to the driver, vehicle, or environment. Therefore, careneeds to be taken in interpreting the results of the exploratory and descriptive analyses of the data or of the clinicalinvestigation. Since NMVCCS only collects data on crashes
that meet the above-mentioned criteria, the estimates basedon NMVCCS data should not be compared with those fromother databases such as NHTSA’s General Estimates System(GES) or the NASS-CDS.

Suggested APA Format Citation for This Document:
Singh, S. (2018, March). Critical reasons for crashes investigated in theNational Motor Vehicle Crash Causation Survey. (Traffic Safety Facts
Crash•Stats. Report No. DOT HS 812 506). Washington, DC:
National Highway Traffic Safety Administration.

This Crash•Stats was prepared by Santokh Singh, Senior
Statistician, Bowhead Systems Management, Inc., working
under contract with the Mathematical Analysis Division of theNational Center for Statistics and Analysis, NHTSA. For questions regarding the information presented in this document,
please contact NCSAWEB@dot.gov.
Published by NHTSA’s National Center for Statistics and Analysis		

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