FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION

FMCSA Contact: Deborah Freund, MC-PSV, (202) 366-5541

Stress and Fatigue Effects of Driving

Longer-Combination Vehicles

Introduction

Driver fatigue is a safety issue of special concern to commercial motor vehicle (CMV)

transportation. Section 4007(d) of the Intermodal Surface Transportation Efficiency Act

of 1991 directed the Department of Transportation to study, among other issues, the

effects of longer-combination vehicle (LCV) operation on driver stress and fatigue. To

accomplish this, the Federal Motor Carrier Safety Administration [formerly the Office 

of Motor Carriers, Federal Highway Administration (FHWA)] sponsored a study to find

out if there were significant differences between the stress and fatigue effects of 

operating single tractor-trailer vehicles and tractors pulling triple-trailer combinations.

The work was carried out in cooperation with the National Highway Traffic Safety

Administration, by the Battelle Human Factors Transportation Center, in Seattle,

Washington.

This study was conducted as a part of a major program by FHWA to investigate 

commercial driver fatigue in CMV operations. This tech brief summarizes the study final

report, Stress and Fatigue Effects of Driving Longer-Combination Vehicles, which will

be available from the National Technical Information Service.

Purpose

This study investigated the possibility that LCVs, with their increased length, greater

weight, and greater number of trailers, could significantly increase the amount of

stress and fatigue experienced by the driver. The study was limited to the issue of

fatigue under proper and normal operating conditions; it was not intended as an

investigation of the safety of these vehicles. 

Methodology

Between October 31, 1994 and January 21, 1995, 24 experienced LCV drivers drove

approximately 2700 mi (4347 km) each in specially equipped and loaded single- and

triple-trailer commercial vehicles under controlled experimental conditions. Tractors

were equipped with video and digital equipment to gather data on the drivers’

performance during the study. Drivers also answered standardized questionnaires 

concerning their perception of stress and fatigue at five scheduled times during the

driving day. 

Drivers were recruited from nearly a dozen companies to participate in the study, and

they represented both large national fleets and smaller regional operations. The ages

of participants ranged from 40 to 62 years; each reported at least 9 years of commercial motor vehicle driving experience. All had recent driving experience with tripletrailer combinations.

Three tractor-trailer combination vehicles were used in the study. Each driver who participated in the study was assigned a specific tractor and drove it for the entire study

week, using it to alternately pull each of the three different trailer configurations:

Tech Brief

The goal of the Federal Motor

Carrier Safety Administration

(FMCSA) is to reduce the 

number and severity of large

truck-involved crashes

through more commercial

motor vehicle and operator

inspections and compliance

reviews, stronger enforcement

measures against violators,

expedited completion of 

rulemaking proceedings, scientifically sound research, and

effective commercial driver’s

license testing, recordkeeping,

and sanctions. The Office of

Research and Technology manages research and technology

development and deployment

programs for the FMCSA.

There are nine major research

and technology focus areas:

crash causation and risk 

factors; regulatory evaluation

and reform; compliance and

enforcement; HAZMAT safety

and cargo tank integrity; 

carrier safety management;

driver training and performance enhancement; driver

alertness and fatigue; driver

physical qualifications; and

car-truck proximity.

Driver alertness and fatigue

primarily supports current 

and future hours-of-service

rulemaking activities, along

with fatigue outreach and

fatigue management 

technology development.

Office of Research and

Technology

400 Seventh Street, SW

MC-RT; Room 3107

Washington, DC 20590
• single 48-foot trailer;

• A triple-trailer combination with three 28-foot 

trailers and standard converter dollies (A- dollies);

and

• A triple-trailer combination with three 28-foot 

trailers and double-drawbar, self-steering converter

dollies (C-dollies).

Standard A converter dollies differ from self-steering

C converter dollies in their design and method of

attachment to the towing trailer (see Figure 1). The

A-dolly consists of a fixed axle arrangement attached

to the towing trailer at a single point, while the 

C-dolly consists of a self-steering axle arrangement

attached to the trailer ahead by two connections

spaced about 4 feet apart. 

Each driver participated in the study for 7 consecutive

days, beginning with a familiarization/training day on

Sunday. All driving was done during daylight hours.

Drivers encountered typical operating conditions and

followed standard operating procedures. Each driving

Conventional “A” Dolly ITR Super-C Dolly

Figure 1. 

Plan view of converter dolly characteristics.

Table 1. 

Fatigue-related measures.

Type Measure Purpose

Self Report Stanford Sleepiness Scale Perceived fatigue 

NASA Raw Task Load Index Perceived workload

Worksafe Australia Questionnaire Perceived change in fatigue level 

Computer Critical Tracking Task Hand-eye coordination

Probe Tests Unprepared Simple Reaction Time Reaction time

Two Finger Tapping Test Motor coordination

Code Substitution Task Perception and short-term memory 

Driving Lane Deviation Squared Changes in lane position weighted to highlight 

large changes

Maximum Lane Deviation Extent of change of lane position

Lane Standard Deviation Variability of lane position

Large Steering Wheel Reversals Number of large corrections to steering 

Physiological Heart Period Physical and mental workload

Heart Period Variability Changes in physical and mental workload

• Steering by single pivot point

• Single connection to lead trailer

• Solid Axle

• Wheels steer individually

• Dual connection to lead trailer

• Self-steering Axle
day consisted of approximately 10 duty-hours, of

which approximately 8 hours were spent operating

the vehicle. The remaining duty-hours involved taking

performance tests, responding to interviews, and

other study-related activities. Drivers were given a 

1-hour lunch break and two 15-minute rest breaks

during the day.

Several different data collection methods were 

utilized during this study (see Table 1). The

researchers collected background information from

driver questionnaires and interviews, field observer

notes, and notes made during informal sessions with

participants. This information was used to help clarify

the more formal data gathered during the study. 

The data were used to answer two questions: 

• Does LCV operation result in a statistically significant increase in driver stress and fatigue when 

compared with single tractor-trailer combinations?

• Are the trailer configuration result contributions to

fatigue and performance excessive in terms of 

criteria for safe operations?

Results

The results of the study provide unambiguous

answers to both of the preceding questions. First,

with regard to triple-trailer versus single-trailer 

configuration differences in fatigue, analyses revealed

a consistent pattern of statistically significant results

across all variables. For example, trailer configurations

and other task demand characteristics consistently

affected performance (lane-keeping measures), 

subjective workload, and fatigue/physiological state.

Configuration results across all variables generally 

followed a single, C-dolly, A-dolly (SCA) pattern, with

C-dolly results intermediate to those for single and 

A-dolly configurations.

Fatigue/physiological recovery and subjective workload results also indicated configuration-related 

carryovers (next-day effects) that followed the SCA

pattern (see Figure 2). Specifically, the day following

the A-dolly showed a relatively high level of carryover

fatigue and resulting workload increase, while the

day following the C-dolly showed relatively neutral

carryover effects. In contrast, positive recovery was

seen the day following the single-trailer configuration. Lane-keeping carryover effects paralleled the

SCA fatigue/physiological state and workload 

patterns. Lane variability significantly increased on

the day following the driving of the A-dolly, increased

moderately the day following the C-dolly, and

decreased on the day following the single.

The study also provided insight regarding configuration contributions to fatigue and performance 

degradations in terms of two criteria for safe 

operation: 1) probability of lane exceedance; and 

2) operator workload criteria. In terms of the first of

these, the results indicated that C-dolly triple-trailer

drivers would experience substantially less fatigue

and a lower probability of lane exceedance than 

drivers of A-dolly triple-trailer configurations.

Secondly, with regard to operator workload, the

results indicated that average triple-trailer drivers,

particularly when driving the A-dolly configuration,

would be expected to experience cautionary-tored-line workload levels during which performance

could decline rapidly to below acceptable levels.

15

10

5

0

- 5

- 10

- 15

- 20

- 25

- 30

Single C-dolly A-dolly

RTXL Adjustment

Figure 2. 

Configuration carryover effect for perceived workload.
Researcher

Battelle Human Factors

Transportation Center, Seattle,

Washington 98105. Contract

No. DTNH22-92-D-07001

Distribution

This Tech Brief is being distributed according to a standard

distribution. Direct distribution

is being made to the Service

Centers and Divisions.

Availability

The study final report will be

available from the National

Technical Information Service,

Telephone: (703) 605-6000

Key Words

truck driver, fatigue, alertness,

CMV driver, vigilance, LongerCombination Vehicle (LCV),

Multi-Trailer Combination

Vehicle.

Notice

This Tech Brief is disseminated

under the sponsorship of the

Department of Transportation

in the interest of information

exchange. The Tech Brief 

provides a synopsis of the

study’s final publication. The

Tech Brief does not establish

policies or regulations, nor does

it imply USDOT endorsement of

the conclusions or recommendations. The U.S. Government

assumes no liability for its 

contents or their use.

Web Site

All FMCSA Tech and Analysis

Briefs may be accessed at:

http://www.fmcsa.dot.gov.

Please send us your comments

or suggestions about the

Tech/Analysis briefs.

Technical Writer

Hallie Magyar, Avalon

Integrated Services.

Together these considerations indicate that triple-trailer drivers generally would be

expected to maintain a higher probability of safe operation with the Super C-dolly configuration than with the standard A-dolly 

configuration (see Figure 3).

Conclusions

Three general conclusions regarding configuration effects can be drawn from the study

results:

1. Configurations and other task demands consistently affected performance, subjective

workload, and fatigue/physiological state following an SCA pattern. The A-dolly 

configuration resulted in the highest level of workload and fatigue, the C-dolly 

configuration resulted in the next highest level, and the single-trailer configuration

resulted in the lowest level of workload and fatigue.

2. Driver differences were prominent in all analyses representing between 32 and 51

percent of the relative mean squares for lane-keeping and workload variables. The

results indicate that some drivers are challenged by the driving task and support

adoption of means to reduce this challenge, particularly when driving triples.

3. Configurations also have consistent carryover effects of physiological effects, driver

subjective workload, and lane keeping performance, and imply that triple-trailer 

drivers generally would be expected to maintain a higher probability of safe 

operation with the Super C-dolly configuration than with the standard A-dolly 

configuration.

July 2000

Publication No. FMCSA-MCRT-00-012 

Federal Motor Carrier Safety

Administration

http://www.fmcsa.dot.gov

TASK DEMAND

CHARACTERISTICS

FATIGUE/PHYSIOLOGICAL

STATE

RESPONSES

OPERATOR

CAPABILITIES

– Configurations

– Tractor

– Segment & Day

– Lane Keeping

– CTT

– RTLX

– Individual

 Differences

– RSA

– THM

Figure 3. 

Conceptual overview of results.