Safe practices for motor vehicle operations ansi asse z15 1

Fleet Safety

for Safety Professionals and Fleet Managers

American Society of Safety Engineers Park Ridge, Illinois USA

Joel M. Haight Ph.D., P.E. Editor

Copyright © 2015 by the American Society of Safety Engineers

All rights reserved.

Copyright, Waiver of First Sale Doctrine

All rights reserved. No part of this work may be r eproduced or transmitted in any form or by a

ny means, electronic or mechanical for

commercial purposes, without the permission in writing from the Publisher. All requests for per

mission to reproduce material from this work

should be directed to: The American Society of Safety Engineers, ATTN: Manager of Tech

nical Publications, 520 N. Northwest Highwa y,

Park Ridge, IL 60068

Disclaimer While the publisher and authors have used th

eir best efforts in preparing this book, they mak e no representations or warranties with respe

ct

to the accuracy or completeness of the conte nts, and specifically disclaim any implied war

ranties of fitness for a particular purpose.

The information herein is provided with the un derstanding that the authors are not hereby e

ngaged in rendering professional or legal serv ices.

The mention of any specific products herein does not constitute an endorsement or recom

mendation by the American Society of Safety

Engineers, and was done solely at the discret ion of the author(s).

Library of Congress Cataloging-in-Publica tion Data

Fleet safety: for safety professionals and flee t managers / Joel M. Haight Ph.D., P.E., editor.

pages cm

Includes bibliographical references and index .

ISBN 978-0-939874-00-2 (alk. paper)

1. Motor vehicle fleets--Safecy measures. 2. Motor vehicle fleets--Safety measures--Econo

mic aspects. 3. Motor vehicle

fleets--Management. I. Haight, Joel M., editor. I I. American Society of Safety Engineers.

TL165.F56 2015

388.3' 40684--dc23

2015004103

Managing Editor: Michael F. Burditt, ASSE

Copy Editor: Jeri Ann Stucka, ASSE

Page Composition: Arnnet Sys terns PLC

Cover: Reed Design Studio

Printed in the United States of America

21 20 19 18 17 16 15 7654321

Chapter4

LEARNING OBJECTIVES

• Be able to describe the dynamics of the fleet vehicle.

• Mathematically determine the safe operations of fleet vehicles.

• Identify safe human factors for fleet operations in different environments.

• Learn defensive driving maneuvers and methods.

• Recognize the safety implications presented by a workspace environment.

• Identify safety criteria for fleet operators and drivers.

• Understand occupant protection and the biomechanics that can cause injuries.

• Learn about safe operations during the material-handling process in fleet operations.

Vehicle Engineering and Ergonomics

Dennis R. Andrews

This chapter contains information relating to the commercial (trucks and buses) motor-vehicle fleet industry and loading and unloading facilities. The information included, while not all-inclusive, was obtained from the industry literature, articles, and the author's experi- ence. The chapter has four major sections: Vehicle Engineering and Tests, Traffic Safety Principles, Vehicle Defensive Driving Tactics, and Ergonomic Issues.

The objective of this chapter is to supply information about and data for the fleet industry as researched by the author. Readers will gain useful new and supplementary knowledge of the fleet industry. The reference section and recommended reading section contain books and articles of interest to motor fleet operators and safety per- sonnel concerning both fleet vehicles and fleet facilities.

The chapter includes information and data on roadway incidents (vehicle accidents), safety, and information for safety programs that will be of interest to fleet owners, fleet safety managers, fleet operations managers, fleet insurance managers, depot operations managers, and anyone else with an interest in motor-vehicle fleet operations and safety.

VEHICLE ENGINEERING AND TESTS

Vehicle Offtracking and Swept-Path Width

Special skills are required to operate commercial and fleet vehicles safely. Large vehicles such as multiaxle trucks and buses operate much differently from passenger vehicles, and their drivers must have special training and maintain concentration while driving to avoid accidents and injury. For example, drivers must learn how to turn and back up articulated vehicles because they require additional space for these maneuvers. Drivers must also be aware of ojftracking, a term used to describe the difference between the radius of the path of the center of the steering axle and the center of the rear axle for box-type trucks. For articulated vehicles such as tractors and trailers, the spac- ing along the longitudinal axles of the hitch point must be considered during low-speed turns because the rear wheels do not follow the same path as the front wheels.

47

48

The swept-path width is the difference between the lat- eral distance of the inside rear wheels and outside front wheels during a turning maneuver for both box-type and articulated vehicles. The radius of the turn determines the offtracking and swept-path width. The offtracking amount is always less than the swept-path width since offtracking is a measurement from the center of the front and rear axles and the swept-path width is a measurement of the distance between the outside front wheel and the inside rear wheel during a turning maneuver. Buses and similar large vehicles have comparable maneuvering movements but smaller space requirements than do artic- ulated vehicles such as tractors and trailers. Formulas are used to calculate the swept-path width and offtracking of large articulated vehicles and box-type trucks or buses. These formulas supply the necessary data for roadway design. They are particularly important for designing off- ramps for interstate highways and turnpikes as well as for training vehicle operators on proper turning maneuvers (see Figures 1 and 2 for more details).

The formula for low-speed offtracking of a standard two-axle, box-type truck with dual rear wheels is

OT (in feet) = r, - r2

where

r1 = the turning radius of the front axle r2 :::: the turning radius of the rear axle

(1)

To determine the rear-axle turning radius use the following formula:

r2 = ✓(r,2 -1 2 ) (2)

where

I = the wheelbase of the vehicle (the distance between the front and rear axles).

Consider a box-truck vehicle with a front-axle turn- ing radius of 50 feet and a wheelbase of 10 feet (/), the radius of the rear axle is 49 feet (r

2 ). The calculated off-

track distance is equal to 1 foot (50 - 49). If the wheels of the rear axle are wider than those of the front axle, an adjustment must be made by dividing the difference in the width of the axles by two and adding that number to the result above. For example, if the outside width of the rear wheels is 8 feet and the outside width of the front wheels is 6 feet, the adjustment is 1 (8 - 6 = 2 a- 2 = 1). Adding this result to the 1 foot of calculated offtracking distance noted above, the offtracking amount is 2 feet. (Fricke 1990, 78-15-78-16). Since offtracking represents the difference in radius between the centers of both axles, the swept-path width is calculated by adding one- half of the width of each axle to the offtracking result.

Fleet Safety

A more complicated approach is necessary when dealing with the offtracking of large articulated tractor- trailers and similar vehicles. The formula for a tractor- trailer with ten wheels (3 axles) is

(3)

where

r1 :::: the radius of the center of the front or steering axle

r, :::: the radius of the center of the rear axle (, = the distance of the fifth wheel (also known as

the kingpin, the point at which the trailer and tractor are connected) on the tractor to the center of the drive wheels of the tractor,

I= the wheelbase of the tractor

I, = the wheelbase from the tractor drive wheels to the rear trailer wheels

If the front-axle turning radius is 41 feet, the trac- tor wheelbase is 12 feet, the trailer wheelbase is 36 feet, and the fifth-wheel offset is 1.2 feet, the offtrack distance would be approximately 25.4 feet (Fricke 1990, 78-18-78-19). As with the box-truck example, the swept path can be calculated by adding one-half the width of both the front and rear axles to the offtrack distance. If the vehicle comprises a tractor and two trailers, also known as doubles, additional data are needed: the rearward overhang of the pinte! hitch (the hitch between the first and second trailer) location, the length of the dolly drawbar (the attachment bar between the first and second trailer), and the wheelbase of the full trailer.

Low-speed offtracking occurs when a combination vehicle makes a low-speed turn-for example a 90-degree turn at an intersection-and the wheels of the rearmost trailer axle follow a path several feet inside the path of the tractor steering axle. Figure 1 illustrates low-speed

1-- MlltflW:lllftCIJfflJt0,fl18111QAIUI - - • • MflUMCIOSYCIIOUCF-~

_.....,--41fltl!..!ia}

. -

FIGURE 1. Low-Speed Offtracking (Source: FHA 2007)

------~

Vehicle Engineering and Ergonomics

offtracking in a 90-degree turn for a tractor-semitrailer. Excessive low-speed offtracking makes it necessary for the driver to swing wide into adjacent lanes when mak- ing a turn to avoid climbing inside curbs, striking curb- side fixed objects or other vehicles. On an exit ramp, excessive offtracking can result in the truck tracking inward onto the shoulder or up over inside curbs. For single trailer combinations, this performance attribute is affected primarily by the distance of the tractor kingpin to the center of the trailer rear axle or axle group. Kingpin setting refers to the truck-tractor fifth wheel connection point for the kingpin, which is located to the front of the semitrailer. For multitrailer combinations the effective wheelbase(s) of all the trailers in the combination, along with the tracking characteristics of the converter dollies, dictate low-speed offtracking. In general, longer wheel- bases worsen low-speed offtracking.

High-speed ojftracking results from the tendency of the rear of the truck to move outward due to the lateral acceleration of the vehicle as it makes a turn at higher speeds. Figure 2 illustrates high-speed offtracking for a standard tractor-semitrailer. The speed-dependent com- ponent of offtracking is primarily a function of the spacing between truck axles, the speed of the truck, and the radius of the turn; it is also dependent on the loads carried by the truck axles and the truck suspension char- acteristics (Fricke 1990).

An Analytical Approach

The Western Uniformity Scenario Analysis (DOT 2004) examines the impact that scenario truck configurations would have on freeway interchanges, at-grade intersec- tions, mainline curves, and lane widths of the current

tll9fl ()9) ,nj .....

1- ~T-.ca.,.CHflll:Of-,1,JYI • • • • NnlTMrm"1'Qllll1U:OfMM;UL&

FIGURE 2, High-Speed Offtracking (Source: FHA 2007)

'

49

roadway system. It determines what improvements would be needed to accommodate the new trucks, and estimates the costs of these improvements. The focus of this research is to compare the new truck configurations with the current tractor-semitrailers and LCVs operating in the scenario states.

Unlike the analysis for the Comprehensive Truck Size and Weight (CTS&D) Study, the base case-vehicle in this analysis varies by state, depending on that state's grand- father laws under the 1991 !STEA freeze (DOT 2000). The chosen base case-vehicle represents the worst vehicle from an offtracking perspective currently allowed on the analyzed roadway segment. For example, if the worst offtracking vehicle currently allowed on the roadway is a Turnpike Double (TPD), then the TPD is used as the base case-vehicle for that road segment; if the Rocky Mountain Double (RMD) is the worst offtracking vehicle, then it is used as the base case-vehicle; and if the 53-foot tractor semitrailer has the worst offtracking, it is the base case- vehicle. Table 1 shows the base case RMD and TPD for each state. This precise framing of the base case-vehicle is an improvement to the CTS&W Study's analysis that used the 48-foot tractor semitrailer at 80,000 pounds as the base case-vehicle for all roads (FMCSA 2000).

Table 2 shows the low-speed offtracking and swept path for the analyzed configurations. The measure is shown for a standard 90-degree, right-hand turn with a 42-foot radius, negotiated at a speed of 5 kilometers per hour. (Note that the CTS&W Study analyzed a 38-foot path radius.) Low-speed offtracking is the one measure where the ST AA Double outperforms all the other configurations. The long TPD with twin 48-foot trailers performs the worst of the vehicles.

TABLE 1

Base Case-Vehicles for the Scenario States

Rocky Mountain Turnpike

State Double Double

Colorado 43.5 + 31 48 +48

Idaho 35 + 20 35 + 20

Kansas 48 + 28.5 45 +45

Montana 38 + 28 45 + 45

Nebraska 38 + 20 38 + 20

Nevada 48 + 28.5 48 + 48

North Dakota 48 + 28.5 48+48

Oklahoma 48 + 28.5 48 +48

Oregon 35 + 20 N/A

South Dakota 48 + 28.5 48 +48

Utah 48 + 28.5 48 + 48

Washington 35 + 20 N/A

Wyoming 38 + 27 N/A

(DOT2000)

'1 -fY'

so

TABLE 2

Low-Speed Offtracking and Swept Path of Vehicles

Performance Data (ft)

Vehicle Low-Speed Swept Description* Configuration** Offtracking Path

Single (53") 3-S2 16.12 24.12

STAA Double (2@28') 2-S 1-2 13.52 21.52

RMD 138', 27') 3-S2-3 1857 26.57

RMD (38', 27') 3-S2-4 22.08 30.08

RMD (38', 27') 3-S2-2 21.54 29.54

RMD (35', 20') 3-52-2 15.78 23.78

RMO (38', 28') 3-S2-4 20.06 28.06

RMD (38', 20') 3-53-2 18.42 2642

RMD (38', 27') 3-S2-4 21.02 2902

RMD (43.5', 31') 3-S2-4 20.78 28.78

RMD (38', 271 3-S3-4 19.13 27.13

RMD (48', 28.5') 3-S2-3 21.87 29.87

Short TPD (2@45') 3-S2-4 27.98 35 98

Long TPD {2@48') 3-S2-4 30.63 38.63

Triple A-Train (3@28') 2-Sl-2-2 20.38 28.38

Triple C-Train {3@28"} 2-S 1-2-2 20.38 28.38

•vehicle description shows the vehicle type where RMD is a Rocky Mountain Double and TPD is a Turnpike Double. The numbers in parenthesis give the length of each trailer.

**The first number in the series indicates the number of axles on the power unit, the next set refers to the number of axles supporting the trailing unit ("s" iindicates it is a semitrailer), and the subsequent numbers indicate the number of axles associated with the remaining trailing unit.

(DOT2000)

- Vehicle Power Requirements

Large vehicles, whether articulated or not, need sufficient power to operate safely on highways and streets so they can maintain a safe highway speed and pass safely. Power is the time rate of doing work, and the maximum po1ver an engine can provide is a measure of its performance capability, The power generated by large vehicles can be determined from the formula (ITE 1990, 60-63):

P = RV+ 3600 (4)

where

P = the power used in kilowatts (1 kW = 1-341 horsepower; 1 hp = 550 foot-pounds per second)

R = the total resistance to motion of the truck and trailer

V = the speed of the vehicle in ft/ s

3600 = a constant representing the seconds in 1 hr

Newton's laws of physics relating to force, mass, and acceleration scientifically demonstrate the need for

Fleet Safety

additional power to haul heavy loads safely, Force is the product of mass and acceleration, and it is necessary to overcome inertia, air resistance, tire and roadway resis- tance, potential energy loss due to the grade or incline of a roadway, and any other conditions that require accelera- tion power, More force is needed to pull an 80,000-pound trailer than to pull a 20,000-pound trailer, especially along very steep, hilly streets in cities such as San Francisco, Seattle, and Pittsburgh, The average passenger vehicle requires less force to accelerate than a heavier vehicle on the same roadway.

The mass-to-power ratio is helpful in detennining and comparing levels of performance. The ratio can be in watts and kilograms or horsepower and foot-pounds, This ratio is important in determining minimum requirements of a selected power plant for a vehicle with known load-carry- ing capabilities, When calculating this type of power ratio, consideration should be given to the power source-----cliesel or gasoline. Diesel engines produce more thrust than gaso- line engines since diesel fuel is ignited by compression, For both maintenance and durability, the overwhelming choice for a commercial vehicle power source is a die- sel engine rather than a gasoline engine. Diesel engines are much more expensive than gasoline engines due to their heavier construction. Power-requirement consider- ations should include vehicle operating ranges, locations, and conditions. Driving in a more mountainous terrain requires more horsepower than driving in a typical inner city unless the city has very steep and hilly streets.

The mass-to-power ratio is the measure of a vehi- cle's ability to accelerate and maintain speed up grades. ~fass can be thought of as an indicator of resistance to motion-the higher the mass-to-power ratio, the less the acceleration performance, and the lower the mass- to-power ratio, the greater the acceleration performance. A typical passenger vehicle's mass-to-power ratio is 1550 kg (3425 lbs) to 140 kW (188 hp) of power, A tractor semi- trailer's is approximately 11,000 kg (24,310 lbs) to 240 kW (322 hp) of power (ITE 1990, 57-60), A typical passenger car has approximately 50 percent of the manufacturer's rated engine power available to travel 100 kilometers per hour (approximately 61 miles per hour); a large truck has approximately 94 percent of the manufacturer's rated engine power available. These estimates are useful in determining maximum acceleration rates and maximum speeds on grades for engine power in relation to engine speed and values of acceleration (ITE 1990, 50-54),

Acceleration is determined by the change in velocity over a period of time and is expressed as feet per second per second, or fps2. Since large vehicles have more mass than average passenger cars, large vehicles accelerate more slowly than passenger cars, As a general rule, the range of acceleration for large, loaded trucks is from 0,3 to 1-6 fps2 (see Figure 3),

Vehicle Engineering and Ergonomics

~ 12 "' ~ uJ 8 ::c -t-

4

o Experimental Doto -Predicted

I !!)9!.- - oo iooo ~ Otf O ~ 0 0 0 0 0 0 Oo~ 0

-1---~ 0 10 20 30 40 50 60 70

DISTANCE {ft) 80

FIGURE 3. Field observations of times for 19.8-m (65-ft) tractor-trailer trucks to clear intersection distances after starting from a stop (Source: Transportation Research Board (TRB) 1997)

Transit Buses

Primary human-factors considerations for public transit buses include the following (Woodson 1992, 85):

1. Driver: Clear visibility in all directions (360 degrees), ability to visually monitor passengers, lack of interior reflection at night, and a comfortable seat for lengthy occupancy. Figure 4 is a recommended layout for a transit bus-driver station. (Woodson 1992, 296).

2. Onboard passengers: Level floor with wide aisles, handrails that are easy to grasp, comfortable seating with sufficient room for knees and elbows, good visibility for seeing stops, air conditioning, minimum noise, and a reasonably comfortable ride.

3. Boarding passengers: Ability to identify oncoming buses from a distance and convenient entry handrails.

4. Service personnel: Convenient access to all maintenance components, especially those requiring frequent service.

The entry threshold for passengers must be low enough so that passengers do not have to stretch to step onto the first step from the ground or curb. It is recom- mended that a ramp sufficient to accommodate wheel- chairs be considered for the main entrance. Aisles should be level; a grade could create a hazard for walking or standing passengers. Passenger seats are not usually fancy on intracity buses, but comfortable seats are required for buses traveling long distances. Seating on long-distance buses should be roomy so passengers don't find it nec- essary to stand or walk in the aisle. Arm rests should

)-

, (x2.H,No/t1'1)

..L__ 7•9111

FIGURE 4. Guidelines for bus-driver-station layout (Source: Woodson 1992, p. 296)

be cushioned and ergonomically designed, and reading lamps and footrests should be provided.

The primary consideration in designing intercity buses is the comfort of passengers taking long rides (Woodson 1992, 86). Intercity bus-seat dimensions can be approxi- mately the same as those for city buses, but intercity bus seats must have headrests and reclining backs. A seat that reclines 30 degrees allows a passenger to lean back far enough to prevent his or her head from falling forward. If a seat is able to recline to 45 degrees, it should be adjusted to a horizontal position, which is more comfortable when pas- sengers stretch their legs. The minimum clearance bet:v.reen the seat back in front of a passenger and the forward por- tion of the passenger's seat is approximately eight inches.

Transit bus companies and those who have the responsibility of locating and posting bus-stop signs must consider and document many issues:

• the safety of passengers entering and exiting the bus

• the impact on parking and adjoining landowners' traffic patterns if the stop is to be located in a business area

51

52

• the positioning of stops near intersections (whether to place the stop on the far side or near side of an intersection or midblock) (It is unsafe for a bus to stop at a stop sign, cross through the intersection, and then stop a second time at a bus stop on the opposite corner.)

• crosswalk safety (Onboard signs should direct passengers to wait until the bus departs before crossing streets, and not to cross in front of the bus. At controlled intersections, stops should be placed at a sufficient distance from crosswalks so that pedestrians are not tempted to enter the roadway from behind a stopped bus.)

• the positioning of bus stops with regard to parking areas (They should not be placed within parking areas since normal traffic may have a tendency to park near or within the bus-stop location and create a safety hazard for passengers exiting the bus) (NJ Transit Corp 1998).

• distance from other bus stops

• signage (Bus-stop signs should not be so large that they block regulatory signs or impair the view of the bus driver or other drivers. Usually local townships have regulations about bus-stop- sign locations, and unless there are strong safety objections, signs should be placed accordingly) (\Voodson 1992, 87).

All transit companies should have their own proce- dures and policies regarding the safety of their riders as well as methods of determining bus-stop locations with the safety of both the riding public and driving public in mind. A valuable source of information about bus-stop placement is "TCRP Report 19: Guidelines for the Loca- tion and Design of Bus Stops" (TCRP 1996). Companies must review and revise policies and procedures as cir- cumstances change. A written policy is an excellent tool for training employees.

Braking Performance

The efficiency of braking by a vehicle is considered its braking performance. How well braking systems perform depends on maintenance, design, and environment. Braking performance (Maj is determined bv vehicle weight, linear deceleration, the braking force of the front and rear axles, aerodynamic factors, and the rate of linear elevation of the roadway. The formula for braking per- formance is (Gillespie 1992, 21--42):

Ma,= -W/g D, = -F,r- I·'., - DA- W sin 0 (5)

Fleet Safety

where

W = the vehicle weight

g = the gravitational acceleration

D, = the rate of deceleration in feet per second

F" = the front-axle braking force

I':, = the rear-axle braking force DA = the aerodynamic drag

q = the uphill or downhill grade

The gravitational acceleration (gJ is constant at 32.2 fps'. The braking force can be determined through indi- ces (tables) or from testing. Aerodynamic drag, which can be found in wind tunnel tables, varies among vehicles depending upon their configuration ( e.g., a Corvette automobile has a lower air drag than a flat-front truck tractor). The angle of rise or fall of a hill can be deter- mined by measurement or by estimating. Most vehicles are equipped with antilock brake systems. Constant pres- sure during deceleration (rather than pumping the brake pedal) will increase the performance of antilock brakes, allowing the driver to maintain control of the vehicle.

A test bus experienced frontal impact three times with three different speeds (see Tables 3 and 4). The vehicle's dimensions were:

• length: 11,000 mm

• width: 2500 mm

• height: 2940 mm

• axle distance: 5570 mm

• front/rear overhang: 2630/2800 mm

Frontal Impact on the Test Bus

Measured Values

Maximum impact force at the

left long1tud1nal beam (k/N)

Maximum impact force at the

right longitudinal beam (k/N)

Resultant impact force (k/N)

Maximum acceleration on the

floor above the CGV (g)

Maximum resultant acceleration

in the Hybrid II head (g)

Measured maximum femur force

1n the Hybrid 11 dummy (kN}

(Source: FMCSA 2003)

Bus Frontal Impact onto

Rigid Wall 3,6 km/h speed

6,98 km/h speed 29,76 km/h speed

180 220 780

160 190 390

320 390 1100

3 4 12

3 10 60

1,1 1.3 1,6

Vehicle Engineering and Ergonomics

TABLE 4

Buses Involved in Fatal Cashes by Operator Type, 1999-2005

Carrier Type Number

School 857

Transit 731

Intercity 83

Charter/Tour 256

Other:

Private company 20

Nonprofit Organization 6)

Government 33

Personal 3

Contractor for school district 40

Other 93

Other subtotal 251

Unknown operator type 74

Total 2252

(Source: FMCSA 2003)

F [kNJ

400

200

F [l

600

400

200

~----- F [kN)

1000

800

600

I 400 ,, 200 ;

a [gl

15

10

0,1

As one can see from Figure 5, in a frontal impact the right and left sides of the bus do not have the same force. Also the floor deceleration peaks at about 15 Gs.

53

Percent Large tractor-trailer combinations have an engine-

braking mechanism that uses the engine to retard the forward motion of the vehicle. This is commonly called a "Jake brake" after the company that invented the system~) a cob Manufacturing. The system works by retarding the speed of the vehicle through the use of the engine's exhaust system, which is able to absorb enough energy to stop a 75,000-pound gross combination vehicle without the use of the service brakes at 19 mph on a 10 percent grade (Fitch 1994, 239-254). The system can be adjusted by the vehicle operator.

3.18

32.5

3.7

11.4

0.9

2.8

1.5

0.1

1.8

4.1

11.1

33

100.0

The t\Vo most common types of brake systems are hydraulic and pneumatic. Hydraulic systems are used on typical passenger vehicles and use brake fluid to activate the brake shoes or calipers to decelerate the vehicle.

impact force on right side

0,2 t [s]

impact force on left side

0,2 t [a]

resultant impact force

0,2 t[s]

deceleration on the floor

0,2 t [•1

FIGURE s. Frontal impact of 1 K 411 bus; impact speed 29,76 km/h (Source: FMCSA 2003)

54

Typical passenger vehicles have antilock brake systems. Hydraulic brake systems must be checked periodically to ensure that fluid has not leaked or dropped to an unac- ceptable level due to worn gaskets or hoses. Hydraulic systems must be cleaned (flushed) periodically and new fluid added, since it is very difficult to stop contaminants from entering the system.

Pneumatic brake systems, also known as air brakes, are usually found on large heavy vehicles, such as tractor- trailers (see Figures 6a-d). Brake lag is a term used to describe the time it takes for a pneumatic brake system to reach full pressure and begin to lock or retard the wheels. In passenger vehicles with hydraulic systems, the brake lag is negligible-approximately 0.1 second (100 milliseconds). Some experts estimate the brake-lag time for tractor-trailer combinations at 0.5 to 1.0 second, while others estimate it at 0.25 to 0.5 second depending upon the number of trailers.

Another safety component that influences braking performance on tractor-trailer combinations and other vehicles with pneumatic brake systems is the slack adjuster. The term slack arfjustment refers to the distance needed to adjust the actuation arm to fully compress the brake lining and the brake drum. If the slack adjustment is not properly set, braking performance will be greatly reduced. There are usually slack adjusters for each braking wheel.

0.35 -ABS disconnected (}=x)

• with ABS, lane A: varying snow/ice

Fleet Safety

The specific ranges stated by the manufacturer should be adhered to when adjustment is done. Some pneumatic brakes have automatic adjusters, but those also must be checked for proper adjustment.

Aerodynamics and Tires

The aerodynamics of a vehicle is important for fuel economy and vehicle control. A person traveling down a roadway in a vehicle and with one hand out the window with the narrow part facing forward, will feel little resis- tance. But if the hand is turned so that the palm is directly forward and into the wind, he will feel greater resistance. This aerotfynamic resistance decreases vehicle fuel mileage. The geometric design of most vehicles used to carry goods, such as semitrailers, is rectangular. The tractor or cab of a tractor-trailer combination, however, usually has a more aerodynamically efficient shape.

Lift resistance-the downward force on a vehicle due to the motion of air over and around it-varies with the geometric design of the vehicle. Lift resistance can eas- ily be understood by watching a drag race or NASCAR race. Dragsters and NASCAR race vehicles have a wing on the rear to keep them on the road at high speeds. A Corvette has a greater lift-resistance force than an SUV or a tractor-trailer combination since the rear of a Corvette

I

0.30 C with ABS, lane B: polished ice (no studs)

~'

iii ~s - 0.25 ,. ,. ., -= .. .. " .. ,. .... .. ;g 0.20 " ,. ~ ., .... r 'cl. '"on 0.15 .. "" > ""'

0.10

• with ABS, lane C: harsh ice (scratched by studs) ,1

' . ~~ ' " ' -

' ., /'

!~'/ C

" . JV - ' ~v,:

_/l 1 .l -

i! y , .... 11 l - ' 0.05

0.05 0.10 0.15 0.20 0.25 0.30 Average deceleration (g) without ABS

FIGURE 6a. Deceleration capacity on 3 snowy/icy lanes (A, B, C) with ABS in function (y) or disconnected (x). Each value represents one tyre type average over all drivers with 95% confidence interval. Nine tyre types on lanes A and C; 6 unstudded types on lane B (Source: Strandberg 1998)

Vehicle Engineering and Ergonomics

~ .. ... = t !5 = = ,! .. .. ~ .. ... .. =

U.lU

0.08

0.06

0.04

0.02

0.00

-0.02

-0.04

-0.06

l§lwithABS

• ABS disconnected

Type of Tyre

Summer Summer Friction Friction Friction No studs Studded Studded Studded new Ref worn worn new '5' new '6' new M+S worn new '8' new '9'

FIGURE 6b. Deceleration difference between certain tyre-ABS configurations and reference summer tyres with ABS. Paired comparisons for each driver on lane A (carrying snow and ice). Average over 24 + 24 + 18 = 66 drivers with 95% confidence interval (Source: Strandberg 1998)

o.os -.---------------------------~

= c:, ':::

0.02

0.01 -1--------------

0.00 +--.----,-+--m--.--+- f -0.01 +----I ~ j -0.02

-0.03 +---------+,-----1 Type of Tyre -0.04 _L ____ _____-:::.__......========--------_j

Summer new Ref

Summer worn

Friction worn

Friction new'5'

Friction new'6'

No studs newM+S

FIGURE &c. Deceleration differences between certain tyre-ABS configurations and reference summer tyres with ABS. Paired comparisons for each driver on lane B (polished ice surface, no studs allowed). Average over 24 + 24 + 18 = 66 drivers with 95% confidence (Source: Strandberg 1998)

55

56

0.08

0.06 l!ilwithABS

• ABS disconnected

0.00 +--~~-1--

Fleet Safety

-0.06 L---------=====----------__J Summer Summer Friction Friction Friction No studs Studded Studded Studded new Ref worn worn new '5' new '6' new M+S worn new '8' new '9'

NOTES: al) New summer tyres-Reference type (same four-wheel individuals as bl and cl). a2) New "friction" tyres. Hysteresis rubber for ice and snow adhesion. Asian

makes. ID no. 6. a3) New unstuddedM+S tyres. Made for studding but without studs. a4) New studded types. 105 studs per type. Same make and type as a3.

IDno.8. bl) New summer types-Reference type (same four-wheel individuals as al

and cl). b2) Worn friction tyres. 5 years old. Tread pattern depth 5 mm.

b3) Worn summer tyres. 5 years old. Tread pattern depth 3-5 mm. 64) New studded tyres. 110 studs per tyre. Same four-wheel individuals as c4.

IDno.9. cl) New summer tyres-Reference type (same four-wheel individuals as al

and bl). c2) New "friction" tyres. Hysteresis rubber for ice and snow adhesion. European.

IDno.5. c3) Worn studded tyres. 5 years old. Tread pattern depth 5 mm. c4) New studded tyres. 110 studs per tyre. Same four-wheel individuals as b4.

ID no. 9.

FIGURE &d. Deceleration differences between certain type-ABS configurations and reference summer tyres with ABS. Paired comparisons for each driver on lane C (harsh ice surface scratched by studs). Average over 24 + 24 + 18 = 66 drivers with 95% confidence interval (Source: Strandberg 1998)

is designed to create a downward airflow. Aerodynamics also affects possible loss of control from strong cross- winds. Vehicles arc aerodynamically efficient for forward movement and are tested in a wind tunnel for centerline forces, but crosswinds are extremely difficult to counter- act (Gillespie 1992, 79-103). Vehicles are not efficiently designed for crosswinds and can be very difficult to control when crosswinds are present. This is especially relevant for tractor-trailers.

Tires are extremely important to safe vehicle move- ment and also affect fuel efficiency. An underinflated tire creates greater rolling resistance and consequently is less fuel efficient. Overinflated tires cause uneven wear; there- fore, the most important factor in tire safety, the depth of tread, is decreased, which may not be noticed until an accident occurs.

Besides inflating tires properly, one must align and balance them in order to maintain safe tire wear and fuel efficiency. Tread depth and wear are very important, since the tread disperses water on roadway surfaces and inhibits hydroplaning and loss of control.

Heavy Vehicle Tire Blowout

Tire blo\vout of mechanical origin involves the condition of the materials (tire, rim) and the quality of the assem- bly. \'ilhile less spectacular than an explosion, the energy released during blowout can lead to significant injuries if people are directly in the projection trajectory of the debris. Four events of a mechanical origin that can cause a tire to blow out are (ASTE 2009):

1. Ompressurization of the tire: Possible causes include:

• poorly adjusted compressor pressure

• pressure-gauge or valve problem

incorrect mounting on the rim and

voluntary overpressurization when seating the tire on the rim.

2. Zipperjailure: A design defect, an overloading, or an impact can cause a weakness, a cracking, or a rupture of the tire carcass (see Figure 7). The result can lead to significant air loss, the projection of tire fragments, and a sudden drop in

Vehicle Engineering and Ergonomics

FIGURE 7. Zipper failure in heavy truck tire blowout (Source: ASTE 2009)

pressure at this location, sometimes accompanied by a mark resembling an unstitched or unzipped fabric. Possible causes include:

• deterioration of the envelope exposing the plys or the belts of the tire to contamination by air or humidity

• mechanical impact that damaged the tire's structure

• driving with an underpressurized tire, below 80% of the recommended pressure

driving with overpressurized tires

• overloading

• loss of mechanical properties due to heat, pyrolysis, or thermo-oxidation

• significant carcass wear

• design defect in the weave of the tire cord

3. Tire-demounting: Tire-demounting occurs when the tire accidentally and suddenly comes off the rim with a violent release of air or other gases from inside the tire. Possible causes include:

• mechanical impact, more or less violent, on the rim or the tire

abnormal wear of the rim (edge)

• deformation of the rim or one of its components following overheating

• incorrect original mounting of the tire

• incompatible parts of the rim (multipiece rim)

dimensional or other incompatibilities of the rim and tire

4. Tire in poor condition or with a structural weakness: A worn tire or even a new one can have a somewhat noticeable structural defect. It may then be unable to withstand normal inflation pressure.

Steady-State Cornering

57

Steacfy-state cornering is a term generally used to describe the handling characteristics of a vehicle. It is important to understand the handling of fleet vehicles since these vehicles operate in all types of environmental conditions. A tractor-trailer's cab has steering in the front axle only; the other axles of the tractor follow. In a turn, the inside front wheel has a greater steering angle than the outside front wheel, and the average of the inside and outside front wheel angles is called the Ackmnan angle. The angle between the heading of the front wheel and the actual travel path of the wheel is known as the slip angle. This angle becomes greater-and the tractor becomes more difficult to control-as the friction value bet\veen the tire and the roadway surface becomes smaller (Gillespie 1992, 54--59). The neutral steering angle is one in which the steering angle is the same as the Ackerman angle. This occurs when the slip angle is the same for both the front and rear tires. Understeen·ng occurs when the front wheels slip to a greater extent laterally than the rear wheels, and oversteen'ng occurs when the rear wheels slip to a greater extent than the front wheels.

Suspension, or weight shift, plays a crucial part in cornering because the movement and displacement of the cargo can greatly affect the steering of the vehicle. Trucks that carry liquids have a baffle system within the tank so that movement of the liquid during cornering is generally stabilized. Federal transportation guidelines pertaining to cargo stabilization and securing have been established to address the issue of weight shift, as well as the possibility of personal injury during tbe unload- ing process. Suspensions are usually a trade-off bet\veen stiffness and the ability to absorb rough roadways. Steer- ing geometry includes the understanding of and proper adjustment relating to the toe-in, caster, and camber of the wheels, especially the wheels on the steering axle.

58

When wheels are adjusted to have toe-in, their front edges are closer together than their back edges. Caster is a back- ward tilting of a wheel in relation to the center of the suspension. Camber refers to the amount that the tops of the wheels tilt outward (Gillespie 1992, 60).

Rearward Amplification

\X!hen a combination vehicle makes a sudden lateral movement, such as to avoid an obstacle in the road, its various units undergo different lateral accelerations. The front axles and the cab exhibit a certain kind of acceleration, but the following trailer(s) have greater accelerations. This has been experimentally verified and quantified. The lateral acceleration of the first trailer may be twice that of the tractor, and the lateral acceleration of a second trailer may be four times as much.

The factors that contribute to increased lateral accel- erations of the trailing units is the phenomenon known as reanvard amplification:

• number of trailing units

• shortness of trailers (longer ones experience less amplification)

• loose dolly connections

• greater loads in rearmost trailers

• increased vehicle speeds

Quantifying rearward amplification in terms of mul- tiples of lateral acceleration is relevant to vehicle design, but is not generally relevant to highway geometric design. The Transportation Research Board (TRB) recommended that a reasonable performance criterion would be that the physical overshoot that a following trailer exhibits dur- ing such a maneuver, relative to its final displaced lateral position, be limited to 0.8 m (2.7 ft) (TRB 1997).

Suspension Characteristics

The suspension of a heavy vehicle affects its dynamic responses in three major ways:

1. determining dynamic loads on tires

2. orienting the tires under dynamic loads

3. controlling vehicle body motions with respect to the axles

Suspension characteristics can be categorized by eight basic mechanical properties (TRB 1997):

1. vertical stiffness

2. damping

3. static load equalization

Fleet Safety

4. dynamic interaxle load transfer

5. height of roll center

6. roll stiffness

7. roll steer coefficient

8. compliance steer coefficient

Rollover

Rollover is a serious problem in commercial trucks that have a high center of gravity. The propensity for rollover greatly increases with the height of the center of mass above the ground. For example, it is widely known and has been demonstrated that SUV s have a high propensity to roll over-approximately five times that of standard passenger vehicles. The problem is exacerbated when quick movements from side to side are performed. Given the size of the typical commercial fleet vehicle, any quick movements can be hazardous and create a catastrophic event. If vehicles are carrying toxic chemicals, the hazard is multiplied many times.

Rollovers can occur if a vehicle attempts to enter a curve at a speed greater than the design speed of the curve. The cross slope or superelevation is usually a posi- tive bank, which helps the vehicle to maintain an upright position in a curve. The radius of the curve and the cross slope are important factors and affect each other depending upon the grade of the road and the speed of the vehicle. Heavy trucks may have a rollover threshold (stability factor) of 0.4 to 0.6; in contrast, a sports car's threshold is 1.2 to 1.7. These values are unitless since they are ratios based on the height of the center of mass and the track width of the vehicles. The formula to determine the stability factor of a vehicle where there is no superel- evation or roadway cross slope is

SF= t/2h

where

SF = the stability factor, t = track width

h = the height of the center of mass

If there is a cross slope, the formula is

SF= (t/2 + ',Jh)/ h

where

(6)

(7)

',I = the roadway's cross-slope angle (Gillespie 1992, 309-317)

Vehicles can also roll over if curbs or other low objects trip them (strike them below the center of mass) as they move laterally. These types of rollovers are

Vehicle Engineering and l!.rgonomics

generally preventable if the driver uses common sense and safe driving techniques.

The following points (lvlcKnight and Bahouth 2009) apply to rollovers:

• Although they account for about a tenth of all large truck crashes, rollovers result from causes that are relatively unique to the vehicle and where it is driven.

• The majority of rollovers occur in curves, primarily on- and off-ramps where misjudgment and being in a hurry lead to speeds that are excessive to the vehicle's high center of gravity.

• Failure to adjust speed to the load and the stability, height, and weight of the load is a cause relatively unique to rollovers.

• Inattention, dozing, and distraction often necessitate sudden course corrections, leading to rollovers. However, they play a smaller role in crashes involving trucks than other vehicles.

• Three control errors that are relatively unique to truck rollovers are turning too sharply, turning too little to remain on the road, and overcorrecting steering errors.

• A quarter of rollovers result from problems over which drivers have no control. Half of those are the fault of other drivers, far less than is the case in other truck crashes.

• Large truck instructional programs could reduce the incidence of rollover by the use of videos to expose truck drivers to situations leading to rollovers and through simulation to help drivers develop avoidance skills without being exposed to danger.

Data on speed- and control-related rollovers are pre- sented in Tables 5 and 6.

Emergency Fleet Vehicles

Although emergency fire vehicles are not usually thought of as fleet vehicles, they have evolved in their own man- ner within the transportation system.

Emergency vehicles such as fire trucks may have specialized equipment such as flashing lights and sirens, may be painted special colors, and may have areas of special reflectivity. Flashing lights were invented to bring attention to persons at a distance that an emergency vehicle was approaching. The flashing intensity, dura- tion, and ability to be detected at a distance are of prime importance. Emergency flashing lights primarily convey the message that drivers must give emergency vehicles the right of way. Since they are used among other lights,

TABLE 5

Speed-Related Rollovers

Cause Number Description

Speed 108 Speed excessive to circumstances

Curves 77 Curves taken at excessive speed

Misjudgment 67 Misjudged speed at which the curve could

be taken

Hurrying 13 In a hurry and disregarded speed limitation

Anger 3 Loss of temper in response to other road users

Oversight Failure to notice speed signs

Loads 26 Not adjusting speed to stability, weight,

height

Brakes 15 Not adjusting speed to known poor braking

Road 11 Not adjusting speed to road conditions

Intersect 10 Not adjusting speed to sharp turn at

intersection

Vehicles 5 Not adjusting speed to vehicles ahead

Tires 3 Not adjusting speed to worn tread

Sight distance 2 Not adjusting speed to limited sight distance

(Source: McKnight and Bahouth 2009)