■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ FHWA-MC-98-008 Technician Guidelines for Antilock Braking Systems Air-Braked Trucks, Tractors and Trailers Prepared for the U.S. Department of Transportation Federal Highway Administration by The Maintenance Council Federal Highway Administration U.S.
■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Technician Guidelines For Antilock Braking Systems Air-Braked Trucks, Tractors, and Trailers ■ ■ ■ ■ ■ ■ ■ ■ Prepared for the U.S. Department of Transportation Federal Highway Administration 400 Seventh Street , S.W. Washington, D.C.
Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No. FHWA-MC-98-008 5. Report Date 4. Title and Subtitle March 1, 1998 Technician Guidelines for Antilock Braking Systems 6. Performing Organization Code 7. Author(s) 8. Performing Organization Report No. The Maintenance Council of the American Trucking Associations 10. Work Unit No. 9.
PURPOSE The purpose of this document is to provide truck technicians with general guidelines for ABS operation, maintenance, inspection and troubleshooting. Technicians should always consult the appropriate vehicle or component manufacturer’s information for specific ABS procedures. DISCLAIMER This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ TABLE OF CONTENTS I. AN INTRODUCTION TO ANTILOCK BRAKING ............ A. What is an ABS? ............................................... B. How Do ABSs Work? ......................................... C. How Should I Drive an ABS-equipped Vehicle During Road Tests? ........................................... D. What Are the Features and Benefits of ABSs? ...
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ 2 ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ I. AN INTRODUCTION TO ANTILOCK BRAKING This section reviews several basic antilock braking system (ABS) concepts.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ pressures to at least one front axle and one rear axle. The ABSs on semi-trailers and dollies must control at least one axle of the vehicle. Additionally, the ABSs on tractors must control one of the rear axles with two modulator valves so that the brake pressure on one end of the axle is independent of the brake pressure on the other end.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ malfunctions in any towed trailer’s or dolly’s ABS. Its location and function are the same as for the powered unit’s ABS malfunction lamp. • Trailer and dolly ABSs—built on or after March 1, 2001—have the equipment needed to send an ABS malfunction signal to the towing vehicle.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ braking/stability. Some vehicles also use a traction control system in conjunction with the ABS. Traction control helps the ABS improve vehicle traction by minimizing wheel slip on the drive axle during acceleration.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ • Operate mixed combination vehicles (with and without an ABS) the same way one would operate totally nonABS combination vehicles. Apply only the brake pressure needed to achieve the desired deceleration while ensuring vehicle stability. Monitor the combination vehicle behavior and back off the brake pedal, if possible, to keep the units under control. D.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ II. ABS COMPONENT DESCRIPTIONS & OPERATION This section describes the design and operation of ABS components. When you complete this section, you should understand the purpose and function of all major ABS parts including: the ECU, the modulator valve, the wheel speed sensor, ABS malfunction/ indicator lamp, ABS diagnostic components, and traction control.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ FIGURE 1: TYPICAL TRACTOR ABS SCHEMATIC ■ 9
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Technicians can communicate with the ECU through a standard SAE J1587 diagnostic connector (See Fig. 1). Technicians can read and clear fault codes stored in the ECU and run various diagnostic tests with this connector. The type of ECU used and its location (in-cab or frame) vary by manufacturer and application.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ • keep the supply port closed to maintain existing pressure, or • open the supply port to allow brake application pressure to increase and repeat the cycle. C. Wheel Speed Sensors Exciter or Tooth Wheel ABS Sensor Pickup The wheel speed sensor has two main components: the exciter and the pickup.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ • four sensors/four modulators (4S/4M), • six sensors/four modulators (6S/4M), and • six sensors/six modulators (6S/6M). Common configurations for trailers are 2S/1M, 2S/2M, 4S/ 2M and 4S/3M. D. ABS Malfunction Indicator Lamps Vehicles required to have an ABS must have ABS malfunction indicator lamps.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ blink code system. Other ABSs may also have light-emitting diode (LED) lamps on the ECU to indicate problems. ABSs used on trailers sometimes have a place to connect an electronic diagnostic tool. The connector is either on a pigtail to the ECU, on the outside of the ECU, or inside the ECU box.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ III. ABS TROUBLESHOOTING, MAINTENANCE & INSPECTION Although an ABS generally requires no routine maintenance, it should be checked periodically like other components of the air brake system. In this section, we review various aspects of ABS troubleshooting, maintenance and inspection.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ CHART 1: GENERAL DIAGNOSTIC PRINCIPLES Begin ➤ ➤ Step 1: Verify Concern ➤ Step 2: Perform Preliminary Checks ➤ Step 3: Refer to Service Information ➤ Step4:4:Perform PerformChecks Systemof Step Checks Electrical, Electronic, Air Systems ➤ No Step 5: Find and Isolate Problems ➤ Problem Isolated? ➤ Step 5a: Re-examine Complaint Yes ➤ Step 6: Repa
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Step 3: Refer to service information. Vehicle manufacturers provide service procedures which must be followed to ensure proper repair.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ manufacturers. (TMC Recommended Practice 129, “HeavyDuty Vehicle Systems Wiring Checks,” is a good source of general information on electrical diagnostic procedures.) ii. Electronic diagnostic procedures To diagnose an electronic system properly, specialized test equipment approved by the electronic system manufacturer may be required.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ After the problem is corrected with the known good component, reconnect the suspect component to make sure the problem returns. Temporarily connecting a known good component, and then reconnecting the suspect component, will help reduce replacement of incorrect components.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Step 7: Clear fault codes. Clear any codes stored in the ECU identifying the problem. Step 8: Implement any possible preventive measures. Review the vehicle maintenance schedule for required service intervals and perform necessary maintenance. Check for other areas of apparent concern and notify the fleet manager—or fix—prior to release of vehicle. 2.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ • Environmental Protection—Whenever a termination is made in a conductor which disturbs the integrity of the insulation on the conductor, measures must be taken to ensure that the termination is not susceptible to moisture damage or other damage which may result from the conductor or termination being exposed to its normal operating environment.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ bodies. These terminations are very common on commercial vehicles and come in a variety of configurations. Multiple connections in one connector body are typical. Also, various types of proprietary terminations on the same vehicle are common. When repairing or replacing these terminations, special techniques are needed.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Whenever an additional grounding point is to be established on the vehicle, consult the vehicle manufacturer to ensure that the planned alteration does not result in an inadequate ground path for other components on the vehicle. This is especially important when establishing a grounding point between chassis and body. c.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 3. Error Detection Methods One ABS benefit is the ability to electronically detect component or system failures. This electronic detection occurs either during self-test checks at start-up, or during continuous passive monitoring.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ CHART 3: SAMPLE ABS MALFUNCTION INDICATOR LAMP (WARNING LIGHT) DIAGNOSTIC FLOW CHART NOTE: The notations to various sections that appear in Chart 3 refer to the source document from which this chart originally appeared—not this manual. Chart 3 represents a typical ABS warning light diagnostic flow chart.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ TABLE 2: COMMON ABS ERRORS AND RESPONSES DETECTED ERROR SYSTEM RESPONSE Component or wiring failure ABS malfunction indicator lamp informs driver of fault. Affected wheel(s) is removed from ABS control and switched to normal braking. The remaining ABS valves may continue providing braking control at the wheels.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ TABLE 3: COMMON ABS SENSOR PROBLEMS AND CAUSES PROBLEM CAUSE Sensor signal is erratic. Damaged tooth wheel, excessive hub runout, and/or sensor gap is too wide. Replace as needed. Check sensor adjustment, resistance. Check wheel bearing adjustment. Open sensor circuit. Damaged sensor, cable or loose cable connections. Replace as needed.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ B. ABS Maintenance and Inspection 1. ABS Sensor Pickup Adjustment CAUTION: Follow all recommended safety warnings and cautions. To prevent eye injury, always wear safe eye protection when performing maintenance or service. Do not work under a vehicle supported only by jacks. Jacks can slip or fall over and cause serious personal injury.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Sensor Pickup Removal/ Installation: Front Axle ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 2. Install the fasteners used to hold the sensor pickup cable in place. 3. Apply lubricant to the sensor spring clip and to the body of the pickup. NOTE: Use a mineral oil-based lubricant that contains molybdenum disulfide.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ Sensor Pickup Removal/ Installation: Rear Axle ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ specification (See lubricant recommendation in previous section “ Sensor Pickup Installation—Front Axle”). 2. Clean and inspect the hole in the mounting block. Install the sensor spring clip. Make sure the flange stops on the inboard side of the vehicle. 3.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ 9. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Modulator Valve Removal and Installation The following removal and installation information is offered as a guideline only. Always refer to the manufacturer’s specific instructions when removing or installing ABS modulator valves. Removal 1. Disconnect the harness connector from the modulator valve. Be careful not to criss-cross wiring. 2.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ IV.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ stop lamp and auxiliary circuit. Industry consensus is that meeting this minimum recommendation will ensure adequate power for trailer ABSs. Additionally, TMC RP 141 recommends that: • Pin 7 of the J560 connector be reassigned as a continuous power circuit, activated when the ignition is on.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ V. GLOSSARY OF ABS TERMS The following terms are used by one or more manufacturers to describe different aspects of ABSs: Antilock Braking System (ABS) A system that monitors and controls wheel speed during braking so as to minimize wheel lockup while maximizing vehicle lateral stability. Plural form—ABSs.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Brake Proportioning The limiting of brake air pressure to a specific axle or tandem to compensate for varying vehicle loading. Brake proportioning is most beneficial during bobtail tractor operation. Braked Wheel Behavior The study of wheel reactions during braking, particularly between the road surface and the tire.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Current Current represents the flow of electrons through a conducting medium, such as copper. Current is measured in amperes or amps and can be derived through the following formula: Amp = Volt/Ohm or I=V/R. Cycle A single sequence of pressure application and release during ABS operation.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Failure Lamp An indicator lamp that indicates ABS operational status. See Malfunction Indicator Lamp. FMVSS Federal Motor Vehicle Safety Standard. FMVSS 121, “Air Brake Systems,” is the regulation that applies to air brakes used on commercial vehicles.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ J1708 An SAE Recommended Practice for serial exchange of vehiclebased, noncritical parametric information. This standard establishes the hardware and protocol requirements for the serial data bus. See J1587.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Power Jackknife A non-braking induced condition whereby the drive wheels of a tractor will spin under engine power, resulting in a loss of lateral stability. Quick-Release Valve A commonly used valve located close to a brake chamber that decreases the time required to exhaust air pressure from it.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Seven-Pin Connector An electrical connector used between units of combination vehicles in North America to conduct electrical power for the Stop Lamps, Turn Signals, Running Lamps, Ground, and Auxiliary (or ABS) circuits.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Stopping Distance The distance required to stop a vehicle. Stopping distance measurements begin when application (control) pressure first begins to increase, and end when the vehicle comes to a complete stop. Tandem Control An ABS design in which the four wheels of the tandem axle are controlled by only one modulator.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Vehicle Power The voltage and current delivered to various electrical and/or electronic devices on a vehicle. Typical vehicle power in North America is 9.0-16.0 volts direct current. European vehicles typically operate from 18.0-32.0 VDC. TMC Recommended Practice 137 establishes a voluntary standard that tractors should deliver a minimum power level of 12.
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ VI. INDEX Antilock Braking System (ABS) ........................... 3 adequate power ................................................. 31-32 benefits ...................................................................... 7 common problems ................................................... 23 common sensor errors ............................................
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Grounding Recommendations .......................... 21 Inspection, ABS .......................................... 27-30 modulator valves ...................................................... 29 J560 ..................................................... 31-32, 36 J1587 ....................................................... 8,9, 36 J1708 .................
Technician Guidelines for Antilock Braking Systems ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Sensor Pickup (Continued) operation ................................................................. 11 problems ............................................................. 25-26 removal............................................................... 27-28 resistance .................................................................
■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Federal Highway Administration 400 Seventh Street, S.W. Washington, D.C. 20590 Publication No.
