FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION

FMCSA Contact: Albert Alvarez (MC-RRR) 202-385-2387

Improving Heavy-Duty Diesel Truck

Ergonomics to Reduce Fatigue and

Improve Driver Health and Performance

Background

The purpose of this study was to measure several factors inside the cab of

heavy-duty diesel trucks (HDDVs) that may affect the health and

performance of long-haul freight truck drivers. The evaluated factors were

noise level, vibration from driver and passenger seats, and in-cab air quality.

The data collected for this study are intended to serve as a baseline for

future similar studies that may determine whether new truck designs impact

these factors for the drivers.

The project also sought to develop a computer model to compare the

dynamic trucks behavior without the expense of road testing and to

investigate the relationship between specific vehicle components and

vibration levels in the cab. Very few studies have been conducted in the

United States on vibration from heavy-duty diesel truck seats.

The focus of the study was the Class 8b category HDDV, defined by the

U.S. Environmental Protection Agency (EPA) as vehicles with gross vehicle

weight rating greater than 27,216 kg (60,000 lb).

Method

The sample was comprised of 27 Class 8b category HDDVs of the latestmodel-year vehicles available at the time of the study from four truck

manufacturers. All measurements were conducted while the trucks were

parked with the engines idling at a truck-stop rest area and during an actual

on-road driving episode that included interstate and State highway driving

over moderately steep and relatively flat terrain. The parked engine-idling

test involved measuring in-cab air quality under several engine/heating,

ventilation, and air-conditioning (HVAC) modes of operation.

The evaluated factors were noise level, whole-body vibration (WBV) from

driver and passenger seats, and the air quality inside the cabin. Noise data

were collected continuously during the on-road driving test by an

integrating, averaging sound-level meter that was attached to the right-hand

arm-rest of the driver seat. WBV data was collected by vibration transducer

pads installed on the driver and passenger seats, and on the cab and truck

body frame. In-cab air quality was evaluated by analyzing concentrations of

carbon monoxide (CO), oxides of nitrogen (NOX), and particulate matter

less than 2.5-μm in aerodynamic diameter (PM2.5). During the parked-idling

test, instruments were located in the bedding or trough area of the sleeper

berth. Vibration data for computer modeling purposes was collected by

several vibration transducers placed on the cab and truck body.

U.S. Department of Transportation

Federal Motor Carrier Safety Administration

T e c h B r i e f

Office of Analysis, Research, and

Technology

1200 New Jersey Ave. SE

Washington, DC 20590

Programs of the Federal Motor Carrier

Safety Administration (FMCSA)

encompass a range of issues and

disciplines related to motor carrier

safety and security. FMCSA’s Office of

Analysis, Research, and Technology

defines a “research program” as any

systematic study directed toward fuller

scientific discovery, knowledge, or

understanding that will improve safety,

and reduce the number and severity of

commercial motor vehicle crashes.

Similarly, a “technology program” is a

program that adopts, develops, tests,

and/or deploys innovative driver and/or

vehicle best safety practices and

technologies that will improve safety

and reduce the number and severity of

commercial motor vehicle crashes. An

“analysis program” is defined as

economic and environmental analyses

done for agency rulemakings, as well

as program effectiveness studies,

state-reported data quality initiatives,

and special crash and other motor

carrier safety performance-related

analyses. A “large truck” is any truck

with a Gross Vehicle Weight rating or

Gross Combination Weight rating of

more than 10,000 pounds.

Currently, the FMCSA Office of

Analysis, Research, and Technology is

conducting programs in order to

produce safer drivers, improve safety

of commercial motor vehicles, produce

safer carriers, advance safety through

information-based initiatives, and

improve security through safety

initiatives. The study described in this

Tech Brief was designed and

developed to support the strategic

objective to produce safer drivers. The

primary goals of this initiative are to

ensure that commercial drivers are

physically qualified, trained to perform

safely, and mentally alert.
Findings

The noise levels inside the cab were reasonably low, most likely the result of insulation

from wall padding and floor carpeting installed in the cabin and sleeper berth areas. The

two main areas of concern are vibration from the seats and the high PM2.5 levels that

drivers may encounter while the trucks are parked with engines idling at truck-stop rest

areas as the drivers sleep in the cabs.

Noise Level

Overall results indicated that the noise levels (measured in the trucks during the on-road

test) were well below the Occupational Safety and Health Administration (OSHA)

permissible exposure level (PEL)—90 dBA—and/or the National Institute for

Occupational Safety and Health (NIOSH) recommended value—85 dBA for an 8-hour

work day.* Additionally, the noise levels in the test trucks did not exceed the Action Level

(AL), which was 50 percent of the maximum PEL. This study tended to show overall

noise levels to be somewhat lower than those usually reported in the manufacturers’

literature, which generally shows the noise levels greater than the AL. Slighter higher

noise levels were measured during interstate travel relative to State highway travel. It is

believed that noise from tires, engine revolutions per minute, and wind impacting the cab

at higher road speeds contributed to the elevated noise levels during interstate driving

because higher road speeds are generally permitted on the interstates. Basically, the noise

levels were similar across the truck manufacturers, although some slight differences were

observed.

Whole-body Vibration (WBV)

Differences between truck manufacturers with respect to vibration were somewhat more

pronounced than the noise-level differences observed between truck manufacturers.

Overall results in all cases indicated that vibration (in units of acceleration) from the seats

was generally below the European Union exposure action level—0.5 m/s2 for an 8-hour

driving day. Exposure limits were exceeded in a few trucks, but for the most part these

were isolated and occurred while the trucks were driven on pavements that were in poor

condition. Significant differences in WBV existed between truck manufacturers, and

between interstate and State highway driving, with the higher WBV occurring on the

highway. The comfort index of the seats, in the majority, fell within the “a little

uncomfortable” region, which is one step removed from the best rating possible (i.e., the

“not uncomfortable” region). Comparison between floor and seat vibrations showed that

the floor vibration was absorbed by the seats before it could be transmitted to the driver.

Air Quality

For the parked engine-idling tests, overall CO, NOX and PM2.5 concentrations were

relatively low inside the cab when the truck engine and the HVAC system were turned off.

Highest CO and NOX concentrations occurred during engine-on and HVAC in

recirculation modes. High PM2.5 concentrations occurred during engine-on and HVAC in

fresh-air mode and during engine-on and fan-off modes. These results suggest that longhaul trucks have a tendency to self-pollute the cab during extended periods of parkedidling. It is believed that the problem of self-pollution and the close proximity of many

trucks idling at the same time in truck-stop rest areas create conditions for certain

*According to Federal Motor Carrier Safety Regulations (49 CFR part 393.94), the interior sound level at the driver’s seating position

must not exceed 90 dBA, as measured when the truck is parked with all doors, windows, and vents closed; all power-operated

accessories turned off; and, with the transmission in neutral, the engine is accelerated to—and stabilized at—either its maximum

governed engine speed if it is equipped with an engine governor, or its maximum rated horsepower. The regulation does not specify a

maximum time-weighted-average dBA level for an 8-hour work day, which was the standard used in this study.
byproducts of engine combustion to enter the cab via the HVAC system or by air infiltration

that occurs naturally from cracks between windows and doors. Self-pollution occurs when a

percentage of the vehicle’s own exhaust enters the cab. The measured concentrations of CO

for all of the engine/HVAC modes of operation were well below the OSHA 8-hour timeweighted average (50 parts per million [ppm] by volume) and should not pose health

problems for the drivers sleeping in cabs during rest periods. However, the measured diesel

PM2.5 concentrations, which are known to cause various respiratory and health problems,

exceeded the EPA National Ambient Air Quality Standards (NAAQS), which are 35 μg/m3

(the 24-hour average) and 15 μg/m3 (the annual average). These results were in line with

similar studies that measured in-cab concentrations during parked-engine idling conditions at

truck-stop rest areas. It should be noted that the NAAQS for PM2.5 for the 24-hour averaging

time is not to be exceeded more than once per year on average over 3 years, and the 3-year

average of the weighted annual mean PM2.5 concentrations from single or multiple

community-oriented monitors must not exceed the annual average.

For the on-road tests, the in-cab concentrations of CO, NOX, and PM2.5 were much lower

than the concentrations measured during the parked-idling tests. Average CO, NOX, and

PM2.5 concentrations during parked idling were approximately 1.3, 4.8, and 4.5 times greater,

respectively, than the average on-road concentrations. This suggests that there is less chance

of a truck self-polluting the cab while it is being driven than while it is parked and idling. Incab concentrations were noticed to be slightly higher, however, while driving on the

interstate, relative to driving on the State highway. This condition is thought to be caused by

the higher vehicle densities normally present on the interstate—a larger number of vehicles

producing a greater quantity of pollutants that enter trucks through their HVAC systems.

Computer Modeling

A computer model was created that can predict the vibration at any selected point on the

tractor, and data collected for this study were used as input to the model. Although the

feasibility of developing a method to validate a computer model of one of the test trucks was

demonstrated, this objective was not achieved due to manufacturers declining to provide

proprietary specifications.

Recommendations

Vibration

While the seats for these test trucks were adequate overall in reducing WBV based on the

comfort index used in the study, there is still room for improvement. Also, it may be

necessary to train drivers in the proper use of the seating adjustments, so that they use the

seat controls to full effect.

The sample of test trucks was relatively small in this study and at least 90 percent of the seats

were manufactured by a single company. Also, the vehicles were lease- or rental-trucks, so it

was likely that the standard (not the optional) seats were installed in the trucks for reasons of

cost. Since a wider range of seats was not adequately represented in this study, it is unknown

if all seats perform identically in the ability to attenuate vibration exposures for the driver.

Therefore, it is recommended that a broader range of trucks be tested in future studies which

will include more seat models from several manufacturers.

Air Quality

Offering a recommendation in this area is somewhat complicated because the most

straightforward solution for improving the air quality in the cabin during periods of parked
December 2010

Publication Number:

FMCSA-RRR-10-010-TB

idling would be simply to prohibit engine idling while a driver is resting

in the sleeper berth. However, to do this would severely limit the driver’s

use of the vehicle’s heat and air conditioning, or comfort appliances such

as a microwave or television, that depend on the idling engine to operate.

A solution might be an electric system that provides external current to

the vehicle so that the on-board appliances can be operated without idling

the truck engine.

The presence of numerous trucks parked and idling together in close

proximity at a truck-stop rest area exacerbates the air-quality problem in

the cabs for the other drivers. Since a possible PM2.5 problem exists, it is

also likely that other gaseous hydrocarbon emissions, such as

formaldehyde or acetaldehyde, may be present in the cab during extended

periods of parked idling. The authors recommend additional research to

better define and identify whether any of these other diesel engine

combustion byproducts are present in the cabin during periods of

extended idling.

Full report title:

Improving Heavy-Duty Diesel Truck

Ergonomics to Reduce Fatigue and

Improve Driver Health and

Performance (Document No.

FMCSA-RRR-10-010)

Authored by:

Joshua S. Fu,

Jimmy Calcagno III,

Wayne T. Davis

Performing Organization:

The University of Tennessee

Dept. of Civil & Environmental

Engineering

1506 Middle Drive;

223 Perkins Hall

Knoxville, TN 37996

Key Words:

CMV, comfort, ergonomics, HDDV,

heavy duty diesel truck, in-cab air

quality, noise level, vibration,

whole-body vibration

Notice:

Tech Briefs are disseminated under

the sponsorship of the U.S.

Department of Transportation

(USDOT) in the interest of

information exchange. Tech Briefs

provide a synopsis of the study’s

final publication. Tech Briefs do not

establish policies or regulations, nor

do they 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

www.fmcsa.dot.gov.