Tech Brief FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION

FMCSA Contact: Albert Alvarez, MC-RRR, 202-385-2387

Highway/Heavy Vehicle Interaction:

A Synthesis of Safety Practice

Background

Highway features and improvements have a very large effect on the safety of

heavy vehicle operations. This synthesis is a comprehensive literature review that

looks at the safety interactions of commercial trucks and buses with these highway

features, and identifies highway upgrades that can be made to improve the safety

of heavy vehicle operations. Information is also based on surveys of State

departments of transportation and the trucking industry. The following factors are

addressed: 

 Physical and performance characteristics of heavy vehicles that 

interact with highways

 Geometric design criteria based on vehicle characteristics

 Traffic control devices and traffic regulations

 Intelligent transportation systems (ITS) and their use to communicate 

more effectively with heavy vehicle drivers and provide real-time 

information concerning safe vehicle operation

The specific issues considered in regards to these factors are those that have a

direct relationship to interactions between heavy vehicles and roadway features,

roadside design features, traffic control devices, or traffic regulations, and those

that have a direct relationship to safety. 

Trucks and buses are larger, heavier, and less maneuverable than passenger cars

and make up an increasingly larger proportion of the traffic on U.S. highways. For

example, on many rural interstate highways, commercial trucks and buses now

make up more than one-third of the traffic stream. Many of the established criteria

for highway design and operation used by highway agencies are based on

interactions between highway features and the vehicles that use the highways. For

most of these criteria, the larger and heavier vehicles have more critical

interactions with highway features than the smaller passenger vehicles. Safe

design and operation of highway facilities requires that these interactions be

understood and incorporated in the formulation of highway agency policies and in

the planning of safety improvements that highway agencies make to the highway

system.

Definitions

For the purposes of this synthesis, heavy vehicles include single-unit trucks and

other motor vehicles with gross vehicle weight ratings in excess of 4,550 kg

(10,000 lb). Commercial buses are those that are designed and used to transport 15

or more passengers (including the driver). Intercity and charter buses are

considered; school buses and local transit buses are not addressed, although many

of the issues discussed may also apply to these vehicles.

Programs of the Federal Motor Carrier

Safety Administration (FMCSA)

encompass a range of issues and

disciplines, all related to motor carrier

and bus safety and security. FMCSA's

Office of Research and Analysis, which

includes the Research, Technology,

and Analysis Divisions, 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 the

agency's rulemakings, as well as

program effectiveness studies, statereported 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 10,001

pounds or greater. 

Currently, FMCSA’s Analysis, Research

and Technology Divisions are

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 improve the safety of

commercial motor vehicles. The

primary goals of this initiative are to

improve truck and motorcoach

performance through vehicle-based

safety technologies and infrastructure.

Office of Research and Analysis

400 Virginia Ave., SW 

Suite 600 

Washington, DC 20024
The American Association of State Highway and Transportation Officials’ (AASHTO) Policy on Geometric Design

of Highways and Streets, commonly called the AASHTO Green Book, identifies configurations used as design

vehicles. These findings are based on a comparison of the AASHTO Green Book with the current literature review

and surveys. 

Physical and Performance Characteristics 

A wide variety of heavy vehicle types operate on U.S. highways, and each type has unique characteristics that

interact with highway features. The understanding of these interactions is important to the safe operation of the

highway transportation system. The physical and performance characteristics of heavy vehicles that interact with

highways include vehicle types and configurations, weights and dimensions, turning radius, offtracking and swept

path width, trailer swingout, braking distance, driver eye height, truck acceleration characteristics, rearward

amplification, suspension characteristics, load transfer ratio, and rollover threshold. 

The literature review led to the recognition of various factors that are affected by or affect physical and performance

characteristics:

 The dimensions of heavy vehicles, particularly the spacing between axles and hitch points and the front and 

rear overhang distances, are primary determinants of the vehicle turning radius, offtracking, and swept path 

width. These vehicle performance measures are, in turn, key factors in the design of various highway 

features to accommodate heavy vehicles.

 Antilock brakes are now required by Federal regulation for all newly manufactured heavy vehicles. The 

antilock brakes used on heavy vehicles must meet a specific performance standard. Braking capabilities of 

trucks have improved to the point that the braking distances of passenger cars and trucks on wet pavements, 

where braking distance is most critical to safety, are now nearly equal.

 Heavy vehicle drivers sit higher than passenger car drivers and, thus, have greater eye heights. As a result, 

truck and bus drivers can see farther than passenger car drivers when approaching vertical sight restrictions, 

such as hillcrests. This may permit truck and bus drivers to see traffic conditions or objects in the road 

sooner and, therefore, begin braking sooner. There is no comparable advantage for truck and bus drivers at 

horizontal sight restrictions.

 Because of their lower acceleration rates and greater length, heavy vehicles take longer than passenger cars 

to accelerate and clear specific conflict zones, such as intersections and railroad-highway grade crossings. 

Heavy vehicle speed maintenance capabilities on upgrades are a function of the vehicle’s weight-to-power 

ratio and the length and steepness of the grade. 

 In trucks with more than one trailer, the second or third trailer may experience higher lateral acceleration 

than the first trailer in lane change or avoidance maneuvers. The maximum desirable lateral displacement of 

a trailer due to this rearward amplification is 0.8 m (2.7 ft).

 Vehicle characteristics related to the dynamic stability of trucks, as represented by load-transfer ratio and 

rollover threshold, include dynamic inter-axle load transfer, height of roll center, roll stiffness, roll steer 

coefficient, compliance steer coefficient, center-of-gravity height, overall weight, and longitudinal and lateral 

weight distribution. 

Highway Geometric Design 

Many highway geometric design criteria are based on vehicle characteristics. In many cases, the characteristics of

trucks and buses are the most critical in defining these design criteria or assessing their appropriateness. Highway

geometric design features whose design is based on consideration of vehicle characteristics include sight distance,

upgrades, downgrades, acceleration lanes, horizontal curves, intersection design, interchange ramps, and roadside

features. 
The literature provided several confirmations, including:

 The current sight distance criteria used in highway geometric design, including stopping, passing, intersection,

and railroad-highway grade crossing sight distance, can reasonably accommodate the current heavy vehicle 

fleet.

 Where long, steep upgrades reduce truck speeds by 16 kph (10 mph) or more, the provision of truck 

climbing lanes should be considered. The AASHTO Green Book presents criteria for determining where truck

climbing lanes are warranted and economically justified.

 Long, steep downgrades present a safety concern for heavy vehicles. If the vehicle service brakes are used 

too often in descending the grade, they may overheat and lose their ability to decelerate the vehicle. Because 

of these risks, highway agencies provide warning signs and roadside brake check areas at the top of some 

downgrades and provide emergency escape ramps for out-of-control vehicles in the middle or lower portion of

some downgrades.

 Acceleration lanes are provided at entrance ramps to major highways to provide a location for vehicles to 

increase their speed before entering the highway. The AASHTO Green Book criteria for the length of 

acceleration lanes appear adequate to accommodate average trucks but may not accommodate the lowest 

performance trucks.

 Horizontal curves designed in accordance with AASHTO Green Book criteria allow heavy vehicles to operate 

at the design speed of the curve with a substantial margin of safety against skidding or rolling over. Skidding 

or rollover should occur only when a heavy vehicle substantially exceeds the design speed of the curve.

 Heavy vehicles are a key consideration in the design of intersections. Intersection features that must consider 

the presence, frequency, and characteristics of heavy vehicles include curb return radii for right turns, storage 

lengths for left-turn lanes, median widths on divided highways, and the offset between opposing left-turn 

lanes.

 Interchange ramps are designed to provide sufficient width for other vehicles to pass a stalled heavy vehicle. 

The design and signing of horizontal curves on ramps is important to their safe operation because safety 

problems may result, as noted above, when heavy vehicles exceed the design speed of a curve. A special 

truck rollover warning sign for use at such locations has been used by highway agencies.

Traffic Control Devices and Traffic Regulations 

Traffic control devices and traffic regulations have an important role in safely accommodating heavy vehicles on the

highway and are used by highway agencies for this purpose, especially at locations where safety problems have

occurred or are anticipated. Traffic control device strategies that have been used, or are being considered, to better

accommodate heavy vehicles on the highway include differential speed limits, road restrictions, exclusive lanes and

roadways, signing for long downgrades, signing and marking of interchange ramps, mitigating the restriction of sign

visibility, and modifying signal timing to better accommodate heavy vehicles.

Many traffic control devices and regulations have been found to have less safety benefits than anticipated:

 Highway agencies have tried to improve traffic operations and safety by restricting heavy vehicle use of 

the left lane or restricting heavy vehicles to use only the right lane on major highways. Most evaluations 

of such lane restrictions have shown no effect on safety, positive or negative. However, a recent test in 

Houston for an 8-month period in one freeway corridor recognized safety benefits from left-lane restrictions 

for heavy vehicles.

 Highway agencies have prohibited truck travel on selected highways for a variety of reasons unrelated to

safety. Naturally, this eliminates truck-related crashes on the facility in question, but no studies for these 

sites have examined the safety impact of truck diversion to other routes.
 Some highway agencies have implemented, and others are considering, 

exclusive truck lanes or exclusive truck roadways on selected facilities. 

No measures of the safety performance of such facilities are available.

 Heavy vehicles, because of their size, can block the view of highway 

signs by other motorists. Highway agencies have developed specific 

methods for dealing with this problem where it occurs, including the use 

of additional advance warning signs, as well as placement of signs 

overhead, and on both sides of the road.

 Heavy vehicles are often a consideration in selecting the length of a 

yellow signal phase and assessing the need for an all-red clearance 

interval at signalized intersections.

Intelligent Transportation Systems Initiatives 

Highway agencies are increasingly using Intelligent Transportation Systems (ITS)

initiatives to more effectively communicate with heavy vehicle drivers and provide

real-time information concerning safe vehicle operation. The types of ITS systems

in current use by highway agencies include warning systems for long downgrades,

dynamic curve warning systems, and improved weigh station operations. ITS

initiatives related to heavy vehicle safety also include on-board vehicle technology,

such as collision avoidance systems for buses. 

Recommendations

The synthesis highlighted the following recommendations:

 The marking of passing and no-passing zones on two-lane highways in

regards to sight distance criteria in the Manual on Uniform Traffic Control 

Devices (MUTCD) should be evaluated to ensure that heavy vehicles use 

them properly.

 Research is needed to determine whether the current AASHTO Green 

Book criteria for acceleration lane lengths at entrance ramps to major 

highways leads to poor safety performance, and whether the design 

criteria for acceleration lane length can be changed in a cost-effective

manner.

 Research on sight restrictions and the benefits of providing offset leftturn lanes to remove them has not been documented.

 Highway agencies need better information on the safety effects of 

differential speed limits.

 Further research based on field trials would be desirable to establish

whether lane restrictions have safety benefits.

 Research is needed to provide safety performance measures to assist

highway agencies in decisions about whether to reduce traffic

congestion by building exclusive truck lanes or roadways.

 New and innovative ITS technologies to improve heavy vehicle safety

should be developed, and the safety effectiveness of both existing and 

new systems should be evaluated and documented.

Availability: The synthesis report

"Highway/Heavy Vehicle

Interaction: A Synthesis of Safety

Practice” is available at the

Transportation Research Board and

can be found at www.trb.org.

Key Words: Bus, CMV, carrier

safety, commercial motor vehicle,

heavy vehicle, intelligent

transportation systems,

motorcoach, truck

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

U.S. DOT 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.

Technical Writer: 

Kirse Kelly

C2 Technologies, Inc.

February 2007

Publication No. FMCSA-MCRRR-07-007