Companion Guide to Video July 2018

www.fhwa.dot.gov/federal-aidessentials

As a matter of public safety and as a way of 

safeguarding vital transportation infrastructure, 

State and local law enforcement agencies 

enforce weight restrictions on trucks and heavy 

vehicles travelling public roads. Under normal 

conditions, vehicles over 40 tons are not 

permitted on interstate highways. Additional 

restrictions apply based on the legal load 

combination, a function of vehicle weight and 

axle spacing. For example, 25 tons is the weight 

limit for a typical three-axle truck on an 

interstate highway, while a typical semi-trailer’s 

limit is 36 tons, and a 4 axle dump truck’s is 27 

tons. State and local laws may address the legal 

load combinations allowed on State and county 

roads by farm vehicles and other types of 

commercial trucks. 

When the drivers of trucks operating within the 

legal weight limits cross a bridge, they are 

relying on the bridge owner—whether a federal 

agency, the State Department of 

Transportation, counties, or municipalities—to 

keep the bridge safe and available for their use. 

All of these bridge owners have critical tasks in 

common. They need to:

• Inspect the bridge for deterioration or 

damage;

• Determine if changes in condition have 

reduced the bridge’s structural capacity 

to safely carry legally permissible loads, 

measured by its load rating; and

• Notify the public of any weight 

restrictions.

The responsibility for restricting loads on local 

bridges may fall on State or local agencies 

depending on agreements between each State 

and local governments it serves.

Here we will review how load ratings are 

determined and the best practices for posting 

truck weight restrictions. 

Federal-aid Program Overview

Bridge Load Rating and Posting

Maintaining public safety through effective evaluation, posting, and 

enforcement.
Federal-aid Essentials for Local Public Agencies

Before a bridge or culvert is constructed, an 

engineer designs its structural elements to have 

capacity to meet anticipated use. After 

construction and as part of every bridge 

owner’s responsibility according to the National 

Bridge Inspection Standards, a bridge is 

inspected once every two years. The inspection 

report documents any deterioration or damage 

that might reduce capacity and may 

recommend an updated load rating analysis. 

By considering various loading patterns, and 

associated uncertainties, a rating analysis is 

developed to determine the truck loads that the 

bridge can carry safely on a regular basis, called 

the inventory level, and the maximum 

permissible loads on the bridge, called the 

operating level. Both ratings can be expressed 

in terms of gross vehicle weight and as a ratio of 

gross vehicle to legal weight, or rating factor. 

For example, for semi use, a county bridge may 

have an inventory level rating of 38 tons and an 

operating rating of 64 tons. As the legal weight 

for a semi is 36 tons – the rating factors are 1.06 

and 1.78 respectively. 

When the operating level rating factor is less 

than 1 for a given legal load combination, it no 

longer has the capacity to carry that full legal 

load. As this puts the safety of drivers at risk, 

the bridge owner must restrict truck weights, 

often through a load posting, until such time 

that structural capacity of the bridge is restored 

or the bridge is no longer able to carry traffic. 

When the operating or inventory rating of a 

bridge is less than 3 tons for any vehicle, the 

bridge must be closed to all traffic.

Like other black and white roadway signs, the 

weight restrictions posted at a bridge are legally 

enforceable. Coordinating a posting with law 

enforcement is an effective method for 

ensuring appropriate use of the bridge and 

safeguarding public safety. 

The loss of access to a bridge or culvert may 

disrupt farming or commercial activities, so 

agencies responsible for posting restrictions 

may wish to adopt additional strategies for 

reducing negative impacts and barriers to 

enforcement. For example, agencies can:

• Identify and share alternate routes 

available to heavy vehicles;

• Use an advance posting so commercial 

vehicles can take an alternate route 

without backtracking; and 

• Communicate the new posting and risk 

of overloading the bridge to the public.

Let’s see how one bridge owner fulfilled their 

load rating and posting responsibility.
Federal-aid Essentials for Local Public Agencies

During the inspection of a county owned truss 

bridge, a certified bridge inspector found a truss 

member had corroded and lost 15 percent of its 

cross-sectional area in a critical location. The 

inspection report recommended a load rating 

analysis.

The county manager hired an engineering 

consultant to perform the analysis. During the 

initial meeting, he explained that the bridge was 

used by farm tractors with loaded grain carts 

during harvest time and that their gross weight 

often exceeded 40 tons which is allowed by the 

county. The engineer incorporated this 

scenario into her model and found the rating 

factor for the tractor and loaded grain carts to 

be 0.85. She recommended restricting 

maximum weights for this special vehicle type. 

Use by typical over-the-road trucks was not 

affected by the load rating.

The county manager met with the county 

magistrate to get a posting order. Once they 

could legally post the bridge, the county 

manager provided a copy of the court order for 

the county sheriff to encourage enforcement 

action and directed the installation of a weight 

restriction sign at the bridge and advance 

warning signs at the intersections nearest the 

bridge. As harvest time was approaching, he 

also identified an alternate route for those 

farmers affected and posted the route on a 

popular community poster board at the local 

farm supply store.

Exceeding the posted weight limit on a bridge 

greatly increases the likelihood of permanent 

bridge damage and bridge collapse. A bridge 

that fails under the load of a heavy vehicle 

poses a serious risk to the driver and the 

traveling public. In addition to the loss of 

equipment, the loss of a bridge, and the access 

that it provides, can negatively affect the local 

economy. 

To effectively manage the risk of a bridge failure 

due to overload, a bridge owner must:

• Have a load rating for all bridges in their 

inventory that considers the current 

condition of the bridge and all legal load 

combinations; 

• Post weight restrictions at any bridge 

that cannot safely carry legal loads; and

• Work with law enforcement to ensure 

weight restrictions are enforced.

As a bridge owner, verify that the load ratings 

for the bridges in your inventory are both 

current and complete and ensure that bridges 

that can’t carry legal loads are appropriately 

posted. If they are not, contact your State 

Department of Transportation for assistance.
Federal-aid Essentials for Local Public Agencies

The content of this 

document is not a 

substitute for 

information obtained 

from State departments 

of transportation, 

appropriate FHWA 

Division Offices, and 

applicable laws. 

Scenarios have been 

simplified for emphasis 

and do not necessarily 

reflect the actual range 

of requirements 

applicable to the 

scenario or this topic. 

This document was 

created under contract 

number DTFH61-13-A00001 by the Federal 

Highway Administration, 

U.S. Department of 

Transportation, and is 

offered to the public to 

heighten and focus 

awareness of Federal-aid 

requirements within the 

local public agencies 

community and 

reinforces the 

importance of these 

necessary policies, 

procedures, and 

practices.

This Companion 

Resource is the script 

content for the video 

production of the same 

name. 

Additional Resources

• FHWA Operations website for truck size and weight as a ready source of 

information on Federal standards and guidelines, state enforcement 

activities, and reporting requirements.

https://ops.fhwa.dot.gov/freight/sw/index.htm

• This FHWA Policy Guidance Center page has technical information for load 

rating provides guidelines for rating calculations and policy implementation

https://www.fhwa.dot.gov/pgc/index.cfm?ddisc=110&dsub=1148

• An FHWA brochure on Understanding Bridge Weight Limits

https://ops.fhwa.dot.gov/publications/fhwahop16068/fhwahop16068.pdf

• FHWA's load rating webpage with information and links to the NBIS 

webpage for load rating information

https://www.fhwa.dot.gov/bridge/loadrating/

• The National Bridge Inspection Standards, 23 CFR 650 Subpart C

https://www.ecfr.gov/cgi-bin/text-idx?node=sp23.1.650.c