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Road Management & Engineering Journal
Copyright © 1997 by TranSafety, Inc.
August 10, 1997
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402

Truck Escape Ramps: Determining the Need and the Location
Appeals Court Reviews "Legal Duty" and "Discretionary Function" in Runaway Ramp Crash in Idaho
Effects of Aging on Older Drivers
Vision and Driving Performance in Older Drivers
Easy Ways to Use Waste Glass as Aggregate
Study Discussed Characteristics of Longer Combination Vehicles (LCVs) in Relation to Roadway Design

Study Discussed Characteristics of Longer Combination Vehicles (LCVs) in Relation to Roadway Design

In their article entitled "Operational Characteristics of Longer Combination Vehicles and Related Geometric Design Issues" (Transportation Research Record 1523), David L. Harkey, Forrest M. Council, and Charles V. Zegeer discussed some performance difficulties and safety concerns posed by longer-combination vehicles (LCVs). LCVs are tractor-trailer configurations that result in greater combined lengths than the more common non-LCV trucks. The greater lengths and weights of these vehicles cause them to perform and handle differently than tractor semitrailers or twin trailers, and these factors may affect traffic safety.

The authors examined the performance of LCVs in the areas of offtracking (when a truck's front and rear wheels do not follow the same path), stability, braking and stopping distance, and speed and acceleration. They argued that "an understanding of how these vehicles operate is required to accommodate them through better geometric designs or regulate them through more stringent laws and better enforcement." In concluding their report, the authors suggested further study of these vehicles, particularly Rocky Mountain doubles, turnpike doubles, and triples.

Figure 1 illustrates the dimensions of several LCVs and of some common non-LCVs.

FIGURE 1: Typical LCV and non-LCV configurations (1)


When a combination vehicle turns through an intersection, negotiates an interchange, or rounds a horizontal curve, it experiences offtracking. Describing offtracking, the authors wrote:

Off tracking is defined as the distance between the path of the front inside wheel and the path of the rear inside wheel as a vehicle traverses a curve or turn. Offtracking is a function of the wheelbases of the tractor and trailers and the number of articulation points. The maximum swept path, another way of expressing the amount of offtracking, is equal to the width of the vehicle plus the offtracking distance. If this maximum swept path is greater than the width of the travel lane, the vehicle will encroach into adjacent lanes, onto the shoulder, or run off the road during the turning maneuver.

Both types of offtracking that may occur are functions of speed. In high-speed offtracking, "the rear wheels track outside the front wheels." In the more frequent low- speed offtracking, "the rear wheels track inside the path of the front wheels." Table 1 below quantifies offtracking by showing high-speed offtracking values and maximum swept paths for 102-inch-wide LCVs rounding a curve with a 600-foot radius at 55 miles per hour.

High-Speed Offtracking and Maximum Swept Path Values for Combinations Negotiating a Curve of Radius 183 m (600 ft) (2)

Truck Combination a Offtracking (m) Maximum Swept Path (m)
Tractor Semi - 14.6 0.16 2.75
Turnpike Double - 14.6/14.6 0.34 2.93
Turnpike Double - 13.7/13.7 0.38 2.97
Rocky Mountain Double - 14.6/8.5 0.41 3.00
Double - 8.5/8.5 0.44 3.00
Rocky Mountain Double - 13.7/8.5 0.44 3.03
Triple - 8.5/8.5/8/5 0.65 3.24
a Numbers indicate trailer lengths (m)
1 m = 3.28 ft

Provided they are positioned properly when entering the curve, the LCVs (as shown by the maximum swept-path values) are able to make the turn safely within a 3.4-meter (11-foot) travel lane. The American Association of State Highway and Transportation Officials (AASHTO) recommends minimum lane widths of 3.7 meters (12 feet) where high truck volumes are combined with high speeds. Under such conditions, high-speed offtracking of LCVs may not be a significant problem. However, where the lane width is less than 3.4 meters (11 feet) or the truck is not positioned properly on entrance to the curve, the truck may encroach onto an adjacent lane.

Table 2 below lists the values for low-speed offtracking and the maximum swept paths for combination vehicles negotiating curves with a radius of 61 meters (200 feet). Table 3 gives those values for curves with a radius of 92 meters (300 ft). Both are typical for interchange loop ramps.

Low-Speed Offtracking and Maximum Swept Path Values for Combinations Negotiating a Curve of Radius 61 m (200 ft) (2)

Truck Combination a Offtracking (m) Maximum Swept Path (m)
Double - 8.5/8/5 0.92 3.51
Triple - 8.5/8.5/8.5 1.34 3.93
Tractor Semi - 14.6 1.46 4.06
Rocky Mountain Double - 13.7/8.5 1.56 4.15
Rocky Mountain Double - 14.6/8.5 1.74 4.33
Turnpike Double - 13.7/13.7 2.26 4.85
Turnpike Double - 14.6/14.6 2.59 5.19
a Numbers indicate trailer lengths (m)
1 m = 3.28 ft

Low-Speed Offtracking and Maximum Swept Path Values for Combinations Negotiating a Curve of Radius 92 m (300 ft) (2)

Truck Combination a Offtracking (m) Maximum Swept Path (m)
Double - 8.5/8/5 0.61 3.20
Triple - 8.5/8.5/8.5 0.88 3.48
Tractor Semi - 14.6 0.98 3.57
Rocky Mountain Double - 13.7/8.5 1.04 3.63
Rocky Mountain Double - 14.6/8.5 1.16 3.75
Turnpike Double - 13.7/13.7 1.49 4.09
Turnpike Double - 14.6/14.6 1.71 4.30
a Numbers indicate trailer lengths (m)
1 m = 3.28 ft

For large-combination vehicles, AASHTO recommends a lane width of 4.9 meters (16 feet) for ramps with the radii shown in Tables 2 and 3. The longer turnpike double is the only LCV that cannot function in this recommended lane width. AASHTO further recommends widening open highway curves to handle the types of vehicles expected to use those roads. The authors suggested that when LCVs operate on roads with moderate to severe curves, pavement widths should be "significantly increased to prevent encroachments into an adjacent lane or dropping a wheel off a pavement edge."

Results from tests evaluating LCV performance on ramps have been mixed. For example, tests in California showed Rocky Mountain doubles ran onto shoulders in many situations, and turnpike doubles experienced problems on interchanges. In Michigan, a 31-meter (100-foot) turnpike double handled 4.9-meter (16-foot) lanes on standard loop ramps without significant difficulty, but the vehicle did run onto the shoulder periodically. This event was not considered problematic; however, if the shoulder had not been paved, offtracking might have been a more significant problem.

In an AASHTO survey based on offtracking and length characteristics, researchers found that fewer than half of urban and rural interchanges could handle 14.6-meter (48-foot) tractor semitrailers. And this number "decreased dramatically" for LCVs. As shown by Table 4 below, with current designs less than 25 percent of urban or rural interchanges could handle turnpike doubles.

Percentage of Interchanges Determined Adequate for Various Truck Configurations by State Departments of Transportation (6)

Type of

Truck Configuration
14.6 m Tractor Semitrailor Rocky Mountain Double Turnpike Double Triple
46% 27% 23% 23%
50% 29% 24% 39%
1 m = 3.28 ft

At-grade intersections pose the most serious problems for offtracking. To accommodate large trucks adequately, AASHTO recommends curb radii of 12.2 meters (40 feet) or more, yet this is less than the minimum design turning radii recommended by AASHTO for tractor semitrailers and turnpike doubles. As a result, AASHTO also recommends "the use of three-centered compound curves or simple curves with tapers" for trucks negotiating sharp turns such as intersections. But some truck combinations, when confronted by a 90-degree intersection with a 13.7-meter (45-foot) curb radius, may "encroach into adjacent lanes on the exiting or receiving leg of the intersection." One study showed that Rocky Mountain doubles and turnpike doubles would be forced into opposing traffic lanes far more often than semitrailers to avoid running over curbs when negotiating right turns at intersections.

Another study revealed a potential safety problems when traffic on the cross street of an intersection without a signal had to be stopped to allow an LCV to turn safely. In fact, responses to an AASHTO survey argued that this failure to maneuver a turn safely at an intersection justifies "severely restrict[ing]" LCVs on interstate roadways.


While LCVs are generally less stable than tractor semitrailers, the magnitude of that instability and the situations in which it occurs remain in question. Typically, LCVs are more likely to roll over and are subject to trailer sway and rearward amplification. In addition, the height of the center of gravity, cargo distribution, type and mechanical condition of connections, number of articulation points, trailer lengths, roadway geometry, speed, and drivers' skill all affect the stability of an LCV. The trailer's length and the number of articulation points have the most effect on stability. In short, the shorter the trailer and the greater the number of articulation points, the less stable the vehicle will be.

Present little evidence suggests an LCV itself has a greater rollover tendency than a conventional tractor-semitrailer; however, an LCV is more likely to be involved in a rollover because of the nature of the connections used between the tractor and the second and third trailers of the vehicle. These connections do not provide the stability a fifth wheel does; therefore, a tractor connected to the trailer by a fifth wheel has more resistance to rollover than the second or third trailers of an LCV have.

Trailer sway, defined as the side-to-side movement of multiple trailers, has not been shown to be a significant factor in LCV stability--except for the triple, which can sway up to one foot and encroach into adjacent lanes. The Rocky Mountain double does sway, but not enough to threaten safety; and the turnpike double sways minimally, if at all.

Lane width affects safety concerns for trailer sway; but if highway designers follow AASHTO's lane-width recommendations and if the LCV driver is competent and experienced, safety concerns decrease significantly. As yet, the amount of trailer sway involved in passing situations has not been determined.

Another factor affecting LCV stability is rearward amplification, which occurs "when the lateral acceleration of the tractor is amplified as it travels toward the rear of the trailer." This amplification is most likely to happen when a driver makes a sudden, unexpected steering movement. Triples have the highest degree of amplification, followed by shorter doubles. Rocky Mountain doubles and turnpike doubles offer more stability during sudden steering movements.


Properly adjusted brakes are a requirement for the safe stopping of all air-braked vehicles. They are even more crucial for LCVs, since LCVs carry heavy loads dispersed among two or three trailers. With properly adjusted brakes, LCVs have more potential braking capacity than conventional tractor semitrailers. However, they also have more brakes that need adjusting, and this has presented a problem for all air-braked vehicles. In Maryland and California, surveys revealed that half of all air-braked vehicles inspected had at least one brake out of adjustment. How LCVs compare with other combination trucks in terms of brake adjustment remains unknown.

Under field observation, tests of emergency braking for LCVs revealed mixed and even conflicting results. One area that suggested safety concerns was an LCV's ability to handle a sudden, unexpected, or emergency stop, particularly in high-traffic urban areas. Car drivers often do not understand the complexities of operating large trucks and frequently cut in front of them--which forces truck drivers into sudden, unexpected braking movements.

Braking tests are also used to measure a vehicle's stopping distance. Again, studies involving LCVs yielded mixed results and have offered no conclusive findings that LCVs are either superior or inferior to conventional tractor semitrailers. The studies did conclude that a driver's skills, the distribution of the load, and the road conditions can all affect an LCV's stopping distance.

With regard to braking, the percentage and length of a downgrade can present safety concerns. Because of their increased weights, LCVs typically must rely more on their brakes than tractor semitrailers or twin trailers. However, with properly adjusted brakes, long, steep downgrades may not present problems, since LCVs have more potential braking capacity than non-LCVs.


Federal mandates place the maximum weight limit for tractor semitrailers and twin trailers at 36 320-kilograms (80,000 pounds), but LCVs often weigh much more than this. Their increased weight means that LCVs must have the horsepower, engine torque, and drive-train efficiency to operate within the stream of traffic without excessive speed differentials that can cause problems for both the LCV and other vehicles on the roadway. Crashes are more likely when LCVs travel under the prevailing speed. For example, when a truck travels 16 km/hr (10 mi/hr) under the prevailing speed, the likelihood for a crash increases by 3.7 times. If the LCV is traveling 32 km/hr (20 mi/hr) under the prevailing speed, the chance of a crash goes up by a factor of 15.5. As a result, AASHTO recommends the addition of climbing lanes when the speed differential rises to 16 km/hr (10 mi/hr) or more. Studies in both California and Utah also revealed that LCVs can have difficulty maintaining speeds when they travel on roads with moderate to severe grades.

LCVs also pose safety problems in other areas. On two-lane roads, their greater lengths force passing vehicles to stay longer in the opposing lane--up to two to three seconds longer when passing a triple as compared with a tractor semitrailer. In addition, LCVs have trouble accelerating and merging with traffic on freeways. Studies showed that LCVs may not gain sufficient speed in the acceleration lane to find a gap to merge with traffic, which disrupts the flow of traffic and creates potential safety problems on the freeway and on the ramp behind the LCV. The LCV's lower acceleration speeds, coupled with its need for more longitudinal space, may also affect the roadway capacity.


Studies have provided a partial picture of the operational characteristics of LCVs, but more studies are needed to complete the picture. Specifically, there is a need for crash research studies and for "operational studies [which] address four specific geometric/operational areas":

  1. A study of offtracking and speed maintenance was recommended on rural roads with severe horizontal and vertical curves. Both the Rocky Mountain double and turnpike double have more low-speed offtracking problems than either the tractor semitrailer or twin trailer. The triple is similar to the tractor semitrailer with respect to low-speed offtracking "but is the worst configuration with respect to high-speed offtracking." In addition, LCVs have problems accelerating and maintaining speeds on upgrades.

  2. The authors recommended a study of LCV operations on rural roads that have passing zones and moderate horizontal curves. The trailer sway of the triple is significant, and it also shows the most high-speed offtracking, up to 2 feet. The increased lengths of LCVs also pose safety concerns when car drivers pass them on two-lane roads.

  3. The authors recommended further study of LCV operations on high-traffic freeways. LCVs pose a number of safety concerns on freeways, including their somewhat restricted ability to accelerate on ramps, merge with traffic, weave among the traffic flow, and make sudden movements to brake or avoid obstacles. Such a study would provide a more comprehensive picture of how LCVs affect safety and capacity on high-use freeways.

  4. A study was recommended of operations at rural and urban intersections. The offtracking problems of LCVs have a significant impact at intersections. Such a study could reveal the size of that impact and differences between LCVs and non-LCVs in relation to lane encroachment at intersections.

Copyright © 1997 by TranSafety, Inc.

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