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«HIGHWAY INFRASTRUCTURE INTRODUCTION Highway infrastructure protection historically has been the primary consideration in determining TS&W limits as ...»

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In the United States, the allowable load on a group of three axles connected by a common suspension system (tridem) is determined by the Federal bridge formula rather than a limit set by law (or regulation). In Europe, Canada, Mexico, and most other jurisdictions, tridem axles are given a specific load limit in the same way the United States specifies single and tandem axle VI-24 limits without direct reference to a bridge formula. This is not to say that these tridem limits are not bridge-related. For example, the tridem limits prescribed by the RTAC, which vary as a function of spacing, are based on bridge loading limitations -- not pavement limitations.

THE GVW LIMIT

The existing legal Federal maximum GVW (cap) limit for the Interstate System is 80,000 pounds, although some States allow truck combination weights above this cap under Federal grandfathering provisions. Axle weight limits and the FBF are designed to protect pavements and bridges, respectively. As such, the cap may not be providing any additional protection to pavements and bridges. Nevertheless, it is important to consider such factors as bridge design loads and criteria, structural evaluation procedures, the age of the existing bridges, and the extent to which increased GVWs would affect the fatigue life of bridges in the United States.

44,000-POUND TRIDEM-AXLE WEIGHT LIMIT Original research done for this Study on the pavement and bridge impacts of tridem axles showed how bridge stresses decrease as the axles in the tridem group are spread apart. This allows more weight to be carried on the tridem group as the axles are spread. The opposite is true for pavement damage. The more the axles are spread, the greater the damage. Therefore, as the axles are spread within the group, the allowable weight must be reduced to hold pavement damage constant.

The tridem-axle weight limit of 44,000 pounds was determined by observing where the curve of the increasing bridge allowable load function crosses the curve of the decreasing pavement load equivalency function (see Figure VI-4). The two curves cross at a spread of 9 feet between the two outer axles which gives 44,000 pounds for both functions. To stop short of 9 feet would require a lower load limit as bridge damage would be greater than at 44,000 pounds. To go beyond 9 feet would increase pavement damage over that at 44,000 pounds.

A 6-axle semitrailer combination is more effective in reducing pavement damage than a 5-axle semitrailer combination with a split tandem (two trailer axles spread apart), which is allowed under the current FBF. Table VI-8 provides the weight limits for a tridem axle between 8 and 16 feet and Figure VI-4 illustrates the impact on pavement and bridges.

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12 6 42 48.6 16 8 40 -----

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USE OF TRIDEMS

The use of tridem axles could increase truck load capacity while reducing pavement damage.30 Many heavy bulk haulers have already switched from 3-axle to 4-axle single unit trucks, and as noted above, significant pavement cost savings may be possible. The 80,000-pound GVW limit poses a constraint on adding axles to 5-axle combinations because the extra axle would reduce the payload.

When viewed using the AASHTO load equivalence factors, combinations with tridem axles generally have much lower pavement costs per ton of freight carried than conventional 5-axle combinations. To illustrate this, as shown in Figur VI-5, a 6-axle tractor-semitrailer loaded to 90,000 pounds with a rear tridem carrying 44,000 pounds produces 2.00 ESALs on flexible pavements and 3.83 ESALs on rigid pavements. The corresponding ESAL values for a conventional 5-axle tractor-semitrailer carrying 80,000 pounds are 2.37 (flexible) and 3.94 (rigid).

Assuming tare weights of 28,000 and 29,500 pounds for the 5- and 6-axle combinations, respectively, and using the AASHTO load equivalence factors, the ESALs per million pounds of payload for the trucks shown in Figure VI-5 are shown in Table VI-9.

Both the TRB Special Report 225 and the AASHTO TS&W Subcommittee suggest consideration of the TTI bridge formula which could allow about 90,000 pounds for a 6-axle tractor-semitrailer combination.

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ELEMENTS OF ROADWAY GEOMETRY AFFECTING TRUCK OPERATIONS

INTERCHANGE RAMPS

Access and exit ramps for controlled access highways are intended to accommodate design vehicles at certain design speeds. Otherwise, trucks heavier than the design vehicle have an increased probability of rolling over, and trucks longer than the design vehicle will have trailer wheels that travel off the pavement to the inside of a curve. The TS&W, configuration, and speed influence the potential for rollover on short loop ramps. The AASHTO policy recommends widening ramps to accommodate combination vehicles. For example, the width of a 1-lane ramp, with no provision for passing a stalled vehicle, would be 15 feet on a tangent section.

The extreme case for design consideration occurs when traffic is congested and stop-and-go conditions exist. The speed component to the offtracking equation is negligible and maximum offtracking to the inside of the curve occurs. Under this condition, the turnpike doubles analyzed in this study offtrack 20 percent more than a 5-axle 53-foot semitrailer combination and as a result, encroach on adjacent lanes or shoulders and necessitate widening beyond AASHTO standards.





INTERSECTIONS

Most truck combinations turning at intersections encroach on either the roadway shoulder or adjacent lanes. For example, the turning path of a truck making a right turn is generally controlled by the curb return radius, whereas the turning path in left turns is not constrained by roadway curbs, but may be constrained by median curbs and other traffic lanes. Combination vehicles with long semitrailers are critical in determining needed intersection improvements to accommodate offtracking requirements. Additionally, the increased time required for a large truck to complete its turn requires longer traffic signals and affects pedestrian safety and intersection efficiency. Figure VI-6 illustrates the intersection maneuver.

Proper design and operation requires that no incursion into the path of vehicles traveling in opposing directions be allowed. A higher standard is often used in design, especially in urban areas, where no incursion into any adjacent lane is allowed. This is particularly critical at signalized intersections where heavy traffic is a prevailing condition. A substantial number of intersections on the existing highway and street network cannot accommodate even a 5-axle tractor-semitrailer combination with a 48-foot semitrailer. Even more intersections would be inadequate to accommodate vehicles that offtrack more than the standard 48-foot semitrailer combination.

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NOTE: Distance from kingping to rear axle is 40 feet; distance from rear axle to rear of trailer is 14.5 feet VI-29 Currently, there are a substantial number of intersections on the highway and street network where improvements for combinations with semitrailers over 48 feet are not feasible and where controls on vehicles, routing, or travel times are needed. Examples of common constraints to intersection improvements are bridges, buildings and sensitive environmental or historic plots. The use of permits in such cases can provide a desirable level of control. Another option for States might be the provision of staging areas where routes and intersections have prohibitive constraints off Interstate-type highways.

CLIMBING LANES

The ability of a truck to maintain speed on a grade is described by the term “gradeability;” the truck’s ability to start on a grade from a standstill is termed “startability.” The ability of various trucks to start and to maintain speeds on grades is a complex subject that primarily depends on net engine horsepower, torque, gearing, drive train efficiency, friction, GVW, and minimum allowable speed. Gradeability and startability are discussed in Chapter 5, Safety and Traffic Operations.

The AASHTO recommends that separate climbing lanes be provided on grades that have substantial truck traffic or that cause typical trucks to slow by more than 10 miles per hour.31

CROSS-SECTION

Cross-section refers to the shape of the surface of the roadway perpendicular to the direction of traffic.32 Under normal operating conditions, cross-section is not a dominant factor in increased TS&W, but under extreme icing conditions, a superelevated cross slope can be a significant problem for vehicles with greater offtracking. The presence of cross-slope discontinuities can also be a problem for vehicles more prone to rollover because of the dynamic forces that they tend to introduce.

HORIZONTAL CURVATURE

The rear wheels of trucks and truck combinations traversing horizontal curves generally offtrack to one side or the other of the paths of the wheels on the steering axle. When a truck is traveling at higher speeds the rear wheels can follow a path outside that of the steering wheels. This effect is relatively small and virtually never results in the need to make geometric improvements beyond those normally made in the design process. On the other hand, when offtracking is to the inside of the curve at lower speeds and in stop-and-go traffic, it is usually more substantial and must be accommodated. Truck combinations with longer trailers are often prone to producing relatively large amounts of offtracking beyond that provided for in AASHTO Substantial is not defined by AASHTO. There is no universally acceptable standard and it is left to the States to define.

The major determinants of the cross section are the number of lanes, the presence of curbing or shoulders, and cross slope. Generally, a slight cross slope is designed into the cross section to assist in proper drainage of precipitation. Often this slope breaks to a steeper slope at the shoulder line, on a divided multilane highway the grade or elevation is generally highest at the centerline.

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VERTICAL CURVE LENGTH

The height of the truck driver's eye is a distinct advantage of trucks over passenger vehicles for crest vertical curves that are designed to maximize stopping sight distance. Vertical curves are generally designed for passenger cars, as a passenger car driver's eye is lower than is a truck driver’s. For a sag vertical curve going from a downgrade to an upgrade, headlight coverage and passenger comfort usually control. The vehicles considered in this study have braking distances similar to vehicles in common use at this time; therefore, no geometric adjustments would be required.

PASSING SIGHT DISTANCES

Distances required for passing trucks can be significantly longer than for automobiles and pickups.

Longer trucks increase the distance required for a car or truck to pass and require more care in order do so safely. Drivers of passenger cars passing trucks, and drivers of trucks who desire to pass other vehicles, are expected to follow the rules of the road and exercise discretion, passing only where sight distance is adequate. On multilane highways, passing is not as critical as passing on a 2-lane highway with traffic in opposing directions. Sight distance criteria for marking passing and no-passing zones on 2-lane highways are more appropriate for a passenger car passing another passenger car: they do not consider trucks, even the standard truck-and 48-foot semitrailer combination vehicle at 80,000 pounds.

The additional lengths of LCVs could require as much as 8 percent more passing sight distance for cars passing LCVs on 2-lane roads; longer and/or heavier trucks would require incrementally longer passing sight distances to pass cars safely on 2-lane roads.

DIMENSIONAL LIMITS IMPACTING TRUCK MANEUVERS

LENGTH LIMITS FOR SEMITRAILERS

The STAA of 1982 requires States to allow the operation of a semitrailer of at least 48 feet long on the NN. All States now allow up to 53 feet on at least some highways. The majority of States prohibit semitrailers longer than 53 feet, the exceptions being Alabama, Arizona, Arkansas, Colorado, Florida, Kansas, Louisiana, New Mexico, Oklahoma, Texas, and Wyoming. 33 Most of these States allow trailers in the 57- to 60-foot range to operate.

Federal Size Regulations for Commercial Motor Vehicles, U.S. DOT, Publication Number FHWA-MC-96

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The STAA of 1982 also established a requirement for States to allow, at a minimum, the operation of two 28-foot trailers (twins) in combination on the Interstate and NN. About one-fourth of the States prescribe 28 feet as a maximum; the others allow additional length up to 30 feet with 28.5 feet being the most common. Prior to passage of the ISTEA, Federal law allowed States to permit longer trailers in combination (commonly referred to as doubles) but did not require States to do so.

OVERALL LENGTH LIMITS

The STAA of 1982 established a prohibition against State laws specifying a maximum length for semitrailer and STAA double combinations operating on the Interstate and NN.

Consequently, most States control total length on the NN by limiting semitrailer and trailer lengths.

About two-thirds of the States have some form of control of total combination length for non-NN highways. While there are no proposals that the Federal law prescribe a total length limit at this time, offtracking standards could effectively limit overall lengths for single- and double-trailer combinations.

VEHICLE WIDTH AND HEIGHT LIMITS

Vehicle widths and heights are important from the standpoint of safety and traffic operations. The effect on roadway geometric design relates to lane and shoulder width and vertical clearances. A 1-lane ramp with a narrow shoulder would result in a blockage if a truck were disabled. Many older structures (overpasses) were constructed with minimal vertical clearances. The addition of pavement overlays over the years may have further reduced these clearances. Increases in vehicle height increases the potential for striking these overhead structures as well as vehicle rollover.



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