FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION

FMCSA Contact: Robert Carroll, MC-RTR, (202) 358-5685

Impact of Sleeper Berth Usage on

Driver Fatigue: Final Report

Introduction

Driver fatigue is recognized as a major factor in accidents involving long-haul truck drivers.

One way in which long-haul truck drivers decrease their fatigue level is through the use of

tractors equipped with sleeper berth units. However even with these units, the quality and

quantity of sleep that a driver obtains may not equal what they would receive if they were

sleeping at home. With that in mind, the primary goal of this project was to assess the

impact that sleeper berth usage has on operator alertness.

Background

To assist the research team in better understanding the issues surrounding sleeper berth

usage, ten focus groups were initially conducted with long-haul operators. (Neale et al.,

1998a and 1998b). These operators included both for-hire and owner-operators, as well as

union and non-union drivers.

The focus groups were held in eight cities across seven states, providing the researchers

with a geographically diverse sample of long-haul drivers. Issues that were explored

included factors affecting the quality and quantity of sleep that drivers receive in sleeper

berths, drivers' physical and mental fatigue while on the road, and other safety-related

issues associated with long-haul truck operations where sleeper berths are used.

Insights gained as a result of these focus groups were broken down into two areas, Team

Driving and Equipment. The research team learned the following:

• Many drivers can only receive quality sleep in a stationary truck, therefore team

drivers should be selected based on their ability to sleep in a moving truck.

• If teaming, drivers should be allowed a voice in selecting their driving partner so that

drivers are teaming with someone they trust.

• In addition, both drivers should know what their schedules are far enough in

advance so as to allow the drivers to come to a consensus as to who will be driving

first. This will allow the partner driving first to prepare by getting adequate sleep

prior to departure.

• Team drivers should be equipped with conventional air-ride tractors (as opposed to

those that are cabover and/or spring-ride).

• Clean sleeper berths are important for drivers who drive different trucks.

• Better noise insulation is needed between the cab and sleeper berth, as well as the

inside and outside of the cab. 

• In addition, better thermal insulation is needed for the tractor.

• Dual escape hatches (on both sides of the sleeper) should be present.

Based on this feedback, and in conjunction with an accompanying literature review,

specific objectives related to issues of quality and quantity of sleep, environmental

variables, types of trips, driver alertness, driving performance, and frequency of accidents,

were written. From this point, researchers at the Virginia Tech Transportation Institute

(VTTI), the Virginia Tech Department of Industrial and Systems Engineering (VT ISE), and

the Harvard Medical School then teamed up to develop the Method and Protocol

described below.

The goal of the Federal Motor

Carrier Safety Administration

(FMCSA) is to reduce commercial

vehicle related fatalities, as well as

the number of persons injured in

commercial vehicle related crashes,

by 50 percent by the year 2010. 

FMCSA's Research and Technology

programs encompass a range of

issues and disciplines, all related to

motor carrier and bus safety and

security. FMCSA defines a

"research program" as any

systematic study directed toward

fuller scientific discovery,

knowledge, or understanding that

will improve safety and security,

and reduce the number and

severity of commercial motor

vehicle crashes. Similarly, a

"technology program" includes

those that adopt, develop, test

and/or deploy innovative driver

and/or vehicle best practices, and

technologies that will improve

safety and security, and reduce the

number and severity of commercial

motor vehicle crashes. 

Currently, FMCSA's Office of

Research and Technology conducts

programs in the area of Driver

Safety Performance, Commercial

Vehicle Safety Performance, Carrier

Compliance and Safety, Safety

Systems and Technology, CrossCutting Safety Initiatives, and

Security. The study described in

the following Tech Brief was

designed and developed as part of

FMCSA's Research & Technology

Driver Safety Performance

Program. The primary goals of this

program are to improve the safety

behavior of non-commercial

drivers in the vicinity of trucks and

buses, and ensure that commercial

drivers are physically qualified,

trained to perform safely, and

mentally alert. 

Office of Research and Technology

400 Virginia Ave, SW

Suite 600

Washington, DC 20024

Tech Brief
Method And Data Analysis

In the study, two Class 8 tractors were set up with the necessary equipment and then loaned to truck drivers

and/or trucking companies to carry their own revenue-producing loads. To ensure that drivers of VTTI's Class 8

vehicles were not stopped by DOT authorities for not complying with Motor Carrier regulations, VTTI obtained

Motor Carrier Operating Authority, the apportioned (IRP) plates, and motor fuel tax permits. (It's important to

point out that due to legal reasons related to the licensing, permitting, and insurance process, participating

drivers represented for-hire companies only; private drivers and owner-operators were not represented in the

study.)

Forty-seven males and nine females participated in this study, constituting 13 teams and 30 single drivers. All

drivers who participated in this study were recruited from one of four for-hire commercial trucking companies,

two that hauled primarily perishable items and used refrigerated trailers, and two that hauled primarily dry

goods and used standard trailers. None of the companies had a union affiliation, and the average age of the

drivers was 43 years of age, with an average of 13 years of driving experience.

The two VTTI-owned tractors included a 1997 Volvo L4 VN-series tractor and a 1995 Peterbilt 379. Functionally

identical instrumentation packages and data collection systems were installed in both trucks. The data acquisition

systems that were installed functioned to record four camera views (shown in Figure 1), including:

• The driver's face

• Driving performance information (including steering, lane departure, and braking)

• Sleeper berth environmental data (including noise, vibration, and temperature)

• Subjective alertness ratings, as well as data from the Nightcap sleep system 

Because the data collection conducted in the Sleeper Berth

project was for periods as long as 6 to 10 days (up to 240

hours), recording data continuously was not possible. As such,

a method for reducing the bulk of the data without losing

information relevant to the goals of the project needed to be

devised. It was therefore decided to implement a data

collection system based on the occurrence of "critical

incidents," and collect vehicle and driver performance data

only when specific triggered events occurred.

For purposes of this study, a "critical incident" was defined as

a measured variable that exhibited a pre-determined

"signature" or exceeded a trigger criterion possibly indicating

fatigue, lapses in performance, a safety-related external event,

or potentially even hazardous driving behavior.

Assisting in this definition were a series of expected criteria which were used to help determine when a critical

incident occurred. The types of triggered events for which the data acquisition system had been programmed

included such things as lane and steering deviations. A complete list of the various trigger types and descriptions

can be found in Table 1. Whenever one of these "triggers" was detected, the video and computer data from a

period of time 1.5 minutes before through 0.5 minutes after the trigger event were automatically saved. The

cause, contributing factors, and circumstances surrounding the incident could then be determined via an analysis

of the videotape after the event occurred.

Severity of the trigger was also assessed and categorized as being either a "driver error without a hazard

present," a "driver error with a hazard present," a "near collision," a "collision," or as being attributable to

another driver. Trigger descriptions included information on driver distraction, driver judgment errors,

impediments in the roadway, and so forth.

Driver interaction with the data collection system was minimal. The drivers' only required activities were to:

• Indicate when an unusual critical incident (as defined above) occurred by pressing a button located on the

dashboard.

• Respond to random inquiries as to their subjective feelings of sleepiness by pressing one of nine buttons on

a panel mounted on the dashboard.

• Wear the Nightcap sleep monitoring device before going to sleep.

• Rate their subjective feelings of sleepiness when they woke up by pressing one of nine buttons on a panel

mounted in the sleeper berth.

• Fill out two paper-based surveys each day.

Figure 1: Four Camera Views
No other operation of data collection hardware was required of the drivers. However, when they were at home

they were also requested to wear the Nightcap sleep monitoring system to collect 2 to 3 nights of home sleep to

be used for comparison. Photos of the instrumentation visible to the driver from the driver's seat and Nightcap

sleep monitoring system can be found in Figure 2 below.

Results And Discussion

In this study, team drivers

were generally very

successful at avoiding

circumstances of extreme

drowsiness. Conversely,

single drivers were greatly

affected by drowsiness,

which in turn,

compromised their ability

to safely operate their

vehicles. 

The benefits of reducing

drowsiness are

highlighted by the team

driving operation. Unlike

extremely tired single drivers who may have felt compelled to continue to drive even when it was dangerous to

do so, the individual drivers in a team operation generally had no similar compulsion to operate the vehicle

when they were extremely tired. From the data collected in this study, it was apparent that the team driving

operation translates into fewer bouts of drowsiness, fewer critical incidents, and, in general, safer trucking

operations.

In addition, team drivers appeared to drive much less aggressively, make fewer errors, and rely effectively on

their relief drivers to avoid instances of extreme drowsiness while driving. In effect, it appeared as though team

drivers undergo a natural "screening" process. This was indicated by a number of the truck drivers during the

focus groups conducted earlier in this project. Drivers indicated that team drivers must be both considerate of

their resting partner and trustworthy with regard to their driving ability. Thus, the level of "acceptance"

necessary to be a successful team driver seems to serve as an effective screening criterion.

Figure 2: Instrumentation Visible to the Driver from the 

Driver's Seat and Nightcap Sleep Monitoring System

Table 1

Trigger Types and Descriptions

Steering Driver turned steering wheel faster than 3.64 radians/sec.

Lateral acceleration Lateral motion equal or greater than 0.3 g.

Critical Incident Button

Lane Deviation

TYPE OF EVENT

Longitudinal acceleration

DESCRIPTION

Acceleration or deceleration equal or greater than 0.25 g.

Activated if the driver crossed the solid lane border (Boolean occurrence).

Activated by the driver upon pressing a button located on the dashboard

when an incident occurred that he/she deemed critical.

Activated if the driver followed the preceding vehicle at a closing rate of 4

seconds or less.

Activated if the Perclos monitor detected that eyes were closed 8.0% of

any given one minute period.

Time-to-Collision

Perclos

Karolinska Sleepiness Rating

Karolinska Sleepiness Rating, 

No Response

Timed Trigger

Lane Departure and Steering

Activated if driver subjectively assessed own drowsiness as extremely

fatigued/difficult to stay awake (rating of 7 or above on sleepiness scale).

Activated if the driver did not respond to the Karolinska rating query.

Baseline data for which the data collection system triggered randomly

every 45 to 75 minutes.

Activated if a lane departure (tractor crossed a lane line) was immediately

followed by a steering event. Disabled if turn signal was activated.
U.S. Department of Transportation

Researcher

This study was performed by the

Virginia Tech Transportation

Institute, 3500 Transportation

Research Plaza, Blacksburg, VA

24061. Contract No. 

DTFH61-96-C-00068.

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 

FMCSA-RT-02-050 is available

from the National Technical

Information Service 

(PB2002-107930).

Key Words

Critical Incident, Driver Fatigue,

Fatigue, Long-Haul, Sleep,

Sleeper Berth, Team Driving

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.

Technical Writer

Craig Steinberg

C2 Technologies, Inc.

August 2002

Publication No. FMCSA-MCRT-02-070

On the other hand, single drivers in the study had many more critical incidents at all levels

of severity as compared to team drivers. Single drivers were involved in four times the

instances of "very/extremely drowsy" observer ratings than were team drivers, and were

more likely to push themselves to drive on occasions when they were very tired. 

In fact, in looking at only the most severe of the critical incidents, more than one-half of

the incidents were actually caused by four single drivers.

In terms of hours-of-service violations, based on a report by Wylie et al., (1996), there were

relatively few instances (about 2.2 percent) of "extreme drowsiness," with most of these

instances being experienced by single drivers, again with a high rate of the occurrence of

this level of fatigue on the second or third shift after the first day of a multi-day drive.

Thus, it appears that the combination of long driving times and multiple days provides the

greatest concern, with several results pointing to the presence of cumulative fatigue. This

means that the length of shifts in the later stages of a trip must also be carefully

considered.

Having mentioned this concern, it's important to point out that critical incidents and/or

driver errors did not increase directly with the hours beyond the regulation. In fact, there

was a substantial decrease in the rate of critical incidents during some of the more extreme

violations. However, one should exercise great caution when interpreting these results to

mean that the hours-of-service should be expanded for the following two reasons:

First, it may be possible that the drivers were making a point to drive more carefully and

cautiously because they were operating outside of the regulation and did not want to get

stopped by law enforcement officials. Alternatively, they may have only risked driving

outside of the regulations because they felt alert and knew that they could continue to

drive safely.

Implications

There were a number of findings as part of this study that indicated that the quality and

depth of sleep was worse on the road, particularly for team drivers. Team drivers have

significantly more sleep disturbances than do single drivers. In addition, for team drivers

who sleep while the vehicle is in motion, factors such as vibration and noise affected their

sleep, although lighting and temperature aspects of the environment did not appear to be

much of a factor.

These findings suggest that while the vehicle was in motion, the noise and motion

environment in the sleeper berth degraded the drivers' sleep. This finding has design

implications for sleeper berths and indicates that when the truck is in motion, greater

attention should be paid to reducing the amount of vibration and noise that invade the

sleeper berth. In particular, more effective noise abatement between the cab and the

sleeper berth could improve sleep quality, perhaps without much additional cost.

Improvement in the vibration/motion environment is a much more difficult problem to

address, but also has the potential to improve sleep if proven practical.

It is also important to note that when environmental factors were considered, it was found

that many of the sleep disturbances that occurred for single drivers could not be attributed

solely to an environmental factor.

Finally, it's worth pointing out that based upon all these results, it is recommended that

research in the areas of driver screening/monitoring, driver training, and fatigue

management be enhanced. Clearly, if systems and procedures could be developed to

identify and either screen or rehabilitate the relatively few drivers that account for the

greatest risk in trucking, the potential exists for a large positive impact in the trucking

industry.

References

Dingus, TA and Neale, VL, et al, Impact of Sleeper Berth Usage on Driver Fatigue, FMCSA

Report No. RT-02-050, Nov. 2001.

Federal Motor Carrier Safety Administration