TRAFFIC SAFETY FACTS
Crash • Stats

DOT HS 812 115

A Brief Statistical Summary

February 2015

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 precrash movement, critical pre-crash event, critical reason, and theassociated factors. A weighted sample of 5,470 crashes was investigated over a period of two and a half years, which represents anestimated 2,189,000 crashes nationwide. About 4,031,000 vehicles,3,945,000 drivers, and 1,982,000 passengers were estimated to havebeen involved in these crashes. The critical reason, which is thelast event in the crash causal chain, was assigned to the driver in
94 percent (±2.2%)† of the crashes. In about 2 percent (±0.7%) of thecrashes, the critical reason was assigned to a vehicle component’s
failure or degradation, and in 2 percent (±1.3%) of crashes, it wasattributed to the environment (slick roads, weather, etc.). Amongan estimated 2,046,000 drivers who were assigned critical reasons,recognition errors accounted for 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 not provide information on pre-crash scenarios and the reason underlying the critical pre-crash events. In 2005, the National Highway Traffic Safety­Administration (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 on-scene data pertaining to events and associatedfactors that possibly ­contributed to crash occurrence. NHTSA’s
National Center for Statistics and Analysis (NCSA) conducted
NMVCCS from July 3, 2005, to December 31, 2007. Crashes wereinvestigated at the crash scene to collect driver, vehicle, and environment-related information pertaining to crash occurrence, witha 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 movement of the vehicle immediately
before the occurrence of the critical event); (ii) critical pre-crash
event (i.e., the circumstance that led to vehicle’s first impact); (iii)
critical reason for the critical pre-crash event (i.e., the immediate
reason for the critical event, which is often the last failure in thecausal chain of events leading up to the crash); and (iv) the crashassociated factors (i.e., the factors that are likely to add to the probability of crash occurrence). This was done with reference to thecrash envelope that comprises of a sequence of events, referring tothe above data elements, which eventually led to the crash.
This Crash•Stats presents some statistics related to one of thefour data elements, namely “critical reason for the critical precrash event.” The data obtained through the sample of 5,470
NMVCCS crashes and the weights associated with them wereused to obtain national estimates of frequencies and percentages
along with their 95-percent confidence limits, as presented in thefollowing 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 thecrash. Although the critical reason is an important part of the descriptionof events leading up to the crash, it is not intended to be interpreted as thecause 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 per crash, based
upon NMVCCS researcher’s crash assessment. The critical reason
was assigned to the driver in an estimated 94 percent (±2.2%) ofthe crashes (Table 1). In addition, the critical reason was assigned
to the vehicle in an estimated 2 percent (±0.7%) and to the environment in about 2 percent (±1.3%) of the crashes.

Table 1. Driver-, Vehicle-, and Environment-Related
Critical Reasons
Critical Reason
Attributed toDrivers
VehiclesEnvironment
Unknown Critical Reasons
Total

Number
2,046,000
44,00052,000
47,0002,189,000

Estimated
Percentage*± 95% conf. limits
94% ±2.2%
2% ±0.7%
2% ±1.3%
2% ±1.4%
100%*Percentages are based on unrounded estimated frequencies(Data Source: NMVCCS 2005–2007)

The critical reasons related statistics are presented in d­ etailin T­able 2 for drivers, Table 3 for vehicles, and Table 4 for­environment.

Critical reason attributed to drivers

95% conf. limits: ± tα/2; deg. freedom × Std. Dev. (α = 0.05, t-value = 2.179)

The critical reason was assigned to drivers in an estimated 2,046,000
crashes that comprise 94 percent of the NMVCCS crashes at thenational level. However, in none of these cases was the ­assignment
intended to blame the driver for causing the crash. The driver-

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related critical reasons are broadly classified into recognition­errors, decision errors, performance errors, and non-­performanceerrors. Statistics in Table 2 show that the recognition error, which
included driver’s inattention, internal and external distractions,and inadequate surveillance, was the most (41% ±2.2%) frequently
assigned critical reason. Decision error 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 11percent (±2.7%) of the crashes, the critical reason was performanceerror such as overcompensation, poor directional control, etc.
Sleep was the most common critical reason among non-performance errors that accounted for 7 percent (±1.0%) of the crashes.
Other driver errors were recorded as critical reasons for about 8
percent (±1.9%) of the drivers.

Table 2. Driver-Related Critical Reasons

Critical Reason
Recognition Error
Decision Error
Performance Error
Non-Performance Error (sleep, etc.)
OtherTotal

Estimated (Based on 94% of theNMVCCS crashes)
Percentage*
Number
± 95% conf. limits
845,000
41% ±2.2%
684,000
33% ±3.7%
210,000
11% ±2.7%
145,000
7% ±1.0%
162,000
8% ±1.9%
2,046,000
100%*Percentages are based on unrounded estimated frequencies(Data Source: NMVCCS 2005–2007)

Table 3. Vehicle Related Critical Reasons
Estimated (Based on 2% ofthe NMVCCS crashes)
Percentage*
Number ± 95% conf. limits
15,00035% ± 11.4%
10,00022% ± 15.4%

Critical Reason
Tires /wheels-related
Brakes-related
Steering/suspension/transmission/
2,000
engine-related
Other/unknown vehicle-related problems 17,000
Total
44,00040% ± 24.0%
100%*Percentages are based on unrounded estimated frequencies(Data Source: NMVCCS 2005–2007)

Critical reason attributed to environment

The critical reason was assigned to about 2 percent of the estimated 2,189,000 NMVCCS crashes. However, none of these issuggestive 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. In about 50 percent(±14.5%) of the 52,000 crashes the critical reason was attributed
to slick roads. Glare as a critical reason accounted for about 17percent (±16.7%) of the environment-related crashes, and view
obstruction was assigned in 11 percent (±7.2%) of the crashes.
Signs and signals accounted for 3 percent (±2.5%) of suchcrashes. In addition, in 52,000 of the crashes with a critical reason attributed to the environment, the weather condition (fog/rain/snow) was cited in 4 percent (±2.9%) of the crashes.

Table 4. Environment-Related Critical Reasons

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 of vehicles
during the NMVCCS on-scene crash investigation; the vehiclerelated critical reasons were mainly inferred through externalvisual inspection of the vehicle components. This resulted inonly mostly external, easily visible factors (tires, brakes, steeringcolumn, etc.) that were cited as the few vehicle-related criticalreasons. The related statistics may not, therefore, be representative of the role of other internal vehicle related problems thatmight have led to the crash. Of the small percentage (2%) of thecrashes in which the critical reason was assigned to the vehicle,
the tire problem accounted for about 35 percent (±11.4%) of thecrashes. 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 coded as critical reasons were assigned inabout 40 percent (±24.0%) percent of such crashes.

3% ± 3.3%

Critical Reason
Slick roads (ice, loose debris, etc.)
Glare
View obstructions
Other highway-related condition
Fog/rain/snow
Other weather-related condition
Signs/signals
Road design
Total

Estimated (Based on 2% of theNMVCCS crashes)
Percentage*
Number± 95% conf. limits
26,00050% ±14.5%
9,000
17% ±16.7%
6,000
11% ±7.2%
5,000
9% (0, 9.9)††%2,000
4% ±2.9%
2,0004% (0.0, 9.1)††%1,000
3% ± 2.5%
1,0001% (0, 3.3)††%52,000100%*Percentages are based on unrounded estimated frequencies††Conf. limits with lower limit 0(Data Source: NMVCCS 2005–2007)

Suggested APA Format Citation for This Document:Singh, S. (2015, February). Critical reasons for crashes investigated inthe National Motor Vehicle Crash Causation Survey. (Traffic Safety
Facts Crash•Stats. Report No. DOT HS 812 115). Washington, DC:
National Highway Traffic Safety Administration.

This Crash•Stats was prepared by Santokh Singh, senior mathematical statistician, Bowhead Systems Management, Inc., working under contract with the Mathematical Analysis Divisionof the National 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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