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of traffic from one type of truck to another, or diversion be- and weight limits must have strong support from elected state
tween truck and rail, if truck size and weight limits were officials.
changed. Even though this study included an extensive review These positions presented in the Western Uniformity Sce-
of past safety studies and developed a consensus of results, nario have not found fulfillment to date.
there was still a lack of confidence in these results. The use of Regional application of size and weight issues is already
crash statistics from operating environments that are signifi- being done successfully through AASHTO subgroups, al-
cantly different from those proposed was a major source of though it can have negative consequences such as heavier
this doubt. This study developed tools to evaluate stability trucks on roads that may not be the safest. Regional permit-
and control properties of different vehicle configurations at ting is discussed in more detail elsewhere.
different weights and dimensions. These tools were intended
to provide a measure of the relative safety compared to vehi-
3.3.7 National Legislative Actions
cles in widespread use. Since the major emphasis of the
CTSW study was the development of analysis tools, it did not By the end of the 1970s, there was something of a patchwork
make strong recommendations for change related to truck of state truck size and weight limits. Not all states allowed dou-
size and weight (68). bles, and overall length limits in others either restricted semi-
trailer length or effectively limited doubles. Some states still re-
tained the 33,240-kg (73,280-lb) gross weight, and axle weights
3.3.6.3 Regulation of Weights, Lengths, and
lower than current federal limits. The 1982 STAA required
Widths of Commercial Motor Vehicles
states to allow larger trucks on the national network, which
The recommendation to form a Commercial Traffic Ef- is comprised of the Interstate system plus the non-Interstate
fects Institute would have, in a general sense, followed the federal-aid primary system. "Larger trucks" included
Canadian example in creating a less political, or nonpolitical
entity, for addressing, in a highly technical manner, issues · Doubles with a trailer length not less than 8.53 m (28 ft);
with truck size and weight. This was not acted upon, so other · A semitrailer not less than 14.65-m (48-ft) long;
recommendations contingent on formation of the Institute · Unlimited length for tractor-semitrailer and double trailer
did not happen. combinations;
It is unclear whether this study was the impetus of some of · Width up to 2.59 m (102 in.);
the research that has ensued, but some of the topics identified · A single axle weight not less than 9,072 kg (20,000 lb), a
in the study have been topics of ongoing research (69). tandem axle weight not less than 15,422 kg (34,000 lb), and
a gross weight determined by Bridge Formula B, up to
36,287 kg (80,000 lb), as is shown in Figure 24 (71).
3.3.6.4 The Western Uniformity Scenario
The Western Uniformity Scenario emphasized findings The current truck weight limits for most major U.S. high-
and conclusions more than recommendations. It advocated ways are based on Bridge Formula B:
developing plans for financing those improvements to infra-
structure, including how the new trucks that are responsi- LN
W = 500 + 12 N + 36
ble for additional costs would contribute to paying those N -1
costs (70).
The Western Uniformity Scenario noted that a recommen- where W = the maximum allowable weight (lb) on the N axle
dation in the TRB review of truck size and weight limits group being considered and L = the extreme axle spacing (ft)
would essentially result in conducting experiments with ve- of this group. Additionally, a maximum gross vehicle weight
hicles that were known not to be safe. This study advocates, of 80,000 lb applies, with a 20,000-lb limit on a single axle and
to the maximum extent possible, giving assurances that the a 34,000-lb limit on a tandem axle with a spread of 4 ft (71).
vehicles to be used would be at least as safe as vehicles on the This formula was recommended in the 1964 House Docu-
road today and that the companies to be operating those ve- ment 354 and was adopted in 1975. The reported basis for this
hicles would have excellent safety records. formula was an allowable overstress of 5% in bridges de-
The Western Uniformity Scenario advocated a regional ap- signed for HS20 loadings and 30% in bridges designed for
proach to truck size and weight change, which could have H15 loadings. A footnote in the House Document also incor-
greater benefits than a series of individual exemptions, but it porated in the resulting legislation prohibited certain vehi-
also could have much more serious adverse consequences un- cles, otherwise legal according to Bridge Formula B, from
less closely monitored. Also, such an approach for a carefully operating on H15 bridges, but these prohibitions are not
controlled and monitored evaluation of changes in truck size often enforced.
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(a) The provisions of the section are applicable to the National System of Interstate and Defense Highways
and reasonable access thereto.
(b) The maximum gross vehicle weight shall be 80,000 pounds except where lower gross vehicle weight is
dictated by the bridge formula.
(c) The maximum gross weight upon any one axle, including any one axle of a group of axles, or a vehicle is
20,000 pounds.
(d) The maximum gross weight on tandem axles is 34,000 pounds.
(e) No vehicle or combination of vehicles shall be moved or operated on any Interstate highway when the
gross weight on two or more consecutive axles exceeds the limitations prescribed by the following formula,
referred to as the Bridge Gross Weight Formula:
except that two consecutive sets of tandem axles may carry a gross load of 34,000 pounds each if the
overall distance between the first and last axle is 36 feet or more. In no case shall the total gross weight of a
vehicle exceed 80,000 pounds.
(f) Except as provided herein, States may not enforce on the Interstate System vehicle weight limits of less
than 20,000 pounds on a single axle, 34,000 pounds on a tandem axle, or the weights derived from the
Bridge Formula, up to a maximum of 80,000 pounds, including all enforcement tolerances. States may not
limit tire loads to less than 500 pounds per inch of tire or tread width, except that such limits may not be
applied to tires on the steering axle. States may not limit steering axle weights to less than 20,000 pounds
or the axle rating established by the manufacturer, whichever is lower.
(g) The weights in paragraphs (b), (c), (d), and (e) of this section shall be inclusive of all tolerances,
enforcement or otherwise, with the exception of a scale allowance factor when using portable scales
(wheel-load weighers). The current accuracy of such scales is generally within 2 or 3 percent of actual
weight, but in no case shall an allowance in excess of 5 percent be applied. Penalty or fine schedules which
impose no fine up to a specified threshold, i.e. , 1,000 pounds, will be considered as tolerance provisions
not authorized by 23 U.S.C. 127.
(h) States may issue special permits without regard to the axle, gross, or Federal Bridge Formula
requirements for nondivisible vehicles or loads.
(i) The provisions of paragraphs (b), (c), and (d) of this section shall not apply to single-, or tandem-axle
weights, or gross weights legally authorized under State law on July 1, 1956. The group of axles
requirement established in this section shall not apply to vehicles legally grandfathered under State groups
of axles tables or formulas on January 4, 1975. Grandfathered weight limits are vested on the date specified
by Congress and remain available to a State even if it chooses to adopt a lower weight limit for a time.
Figure 24. Excerpts of code of federal regulations part 658. Source: Reference (71).
The STAA set minimum limits, and most states chose not · On the overall length of the cargo carrying units of com-
to exceed them, bringing a significant measure of unifor- bination vehicles with two or more such units where one
mity to the truck traffic on the National Network. It did or both exceed 8.69 m (28.5 ft) in length on the national
nothing for other vehicles that operated on state roads network.
under state legislation, and states increasingly exploited
their grandfather rights to allow more and different vehicles The LCV freezes applied to combination vehicles in actual
on more roads. The ISTEA of 1991 therefore imposed two and legal operation in a state on June 1, 1991, and the routes
separate freezes: and conditions in effect on that date were also frozen for ve-
hicle combinations subject to the freeze (73).
· On the maximum weight of LCVs, which consist of any National policy on truck size and weight has not changed
combination of a truck tractor and two or more trailers or significantly since the 1991 ISTEA, except that a small number
semitrailers operating on the Interstate system at a gross of specialized vehicle configurations have been added to the
weight over 36,287 kg (80,000 lb). regulation, and details for some of these have been amended.
