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
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.
BRAKING AND STOPPING DISTANCE
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
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 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.
SPEED AND ACCELERATION
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.
CONCLUSIONS AND RECOMMENDATIONS
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
- 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
- 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.
- 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
- 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.