FHWA Guardrail: MASH Standards, Barrier Types, and Terminals

FHWA guardrail standards require that any guardrail installed on a federally funded road pass full-scale crash testing under the AASHTO Manual for Assessing Safety Hardware, known as MASH, and receive a federal-aid reimbursement eligibility letter from the Federal Highway Administration before a state can install it and be paid back.¹ FHWA writes and enforces the performance rules; state Departments of Transportation choose the hardware from FHWA’s eligible list and handle installation and upkeep.²

What MASH Requires

MASH is published by the American Association of State Highway and Transportation Officials and is the sole crash-testing standard for guardrails and other roadside safety hardware on federal-aid projects. It replaced NCHRP Report 350, which had been the benchmark since the mid-1990s, because cars and trucks on U.S. roads had grown heavier and taller than the Report 350 test vehicles.³

MASH swapped in heavier test vehicles: a 2,425-pound small car in place of the 1,808-pound Report 350 car, a 5,000-pound pickup truck in place of the 4,409-pound one, and a roughly 22,000-pound single-unit truck in place of the 17,600-pound model. The pickup test vehicle must have a center of gravity at least 28 inches high, which reflects the prevalence of SUVs and light trucks. A guardrail that passed Report 350 will not necessarily perform acceptably against the vehicles MASH uses.

Test Levels

MASH sorts crash tests into six Test Levels, each combining a vehicle size with an impact speed. Higher levels simulate more severe crashes and are required on faster or heavier-traffic routes.

TL-1: 31 mph, for very low-speed roads.
TL-2: 44 mph, for low-speed urban and suburban roads.
TL-3: 62 mph with a 2,425-pound car and a 5,000-pound pickup, the common level for standard highway guardrails.⁴
TL-4 and TL-5: 56 mph and 50 mph, using trucks over 17,000 pounds for routes with heavy commercial traffic.
TL-6: the highest level, for tanker trucks and other very heavy vehicles near critical infrastructure.

Most standard highway guardrails are tested to TL-3. Concrete median barriers, bridge rails, and barriers near critical locations such as fuel storage often require TL-4 or higher.

How a Guardrail Gets Approved

A guardrail system must undergo full-scale crash testing at an accredited facility following MASH protocols. Computer modeling and engineering analysis alone will not earn approval, and modified versions of previously tested devices need fresh crash testing under MASH.³

For a TL-3 test, a real 2,425-pound car and a real 5,000-pound pickup are each driven into the barrier at 62 mph and a 25-degree angle.⁴ Researchers then evaluate results in three areas:

Structural adequacy: whether the barrier did its intended job, whether that means redirecting the vehicle, bringing it to a controlled stop, or letting it break through safely. Measurements include how far the barrier deflected and the working width the vehicle occupied.
Occupant risk: the forces experienced inside the vehicle, along with checks for dangerous intrusion into the floor pan or windshield.
Post-impact behavior: where the vehicle goes after impact. A clean redirect is ideal; pocketing, spinning, or vaulting the barrier are failures, and a high exit angle is dangerous because it can send the vehicle into oncoming traffic.

When a device passes, the manufacturer sends its crash-test report to FHWA and requests a federal-aid reimbursement eligibility letter. If FHWA agrees the testing meets MASH criteria, it issues the letter and posts it on the Hardware Eligibility Letters page.⁵ State DOTs use that list when selecting hardware. A device without an eligibility letter cannot receive federal reimbursement, which effectively keeps it off most major highway projects.⁶

Approved Barrier Types

Different road environments call for different barrier designs. Each type has its own deflection characteristics and space requirements.

Semi-Rigid: W-Beam Guardrail

The W-beam guardrail is the familiar steel rail seen along most U.S. highways. The current standard version is the Midwest Guardrail System (MGS), mounted at a 31-inch rail height. FHWA recommends the 31-inch MGS over the older 27-inch design because it performs better against the taller, heavier vehicles now on the road.¹ The MGS uses steel posts with 12-inch-deep blockouts that create space between the rail face and the posts, so the rail can flex on impact and redirect the vehicle rather than snag it. Testing shows the system performs within a height range of roughly 28 to 32 inches, giving agencies tolerance during installation and resurfacing.

Flexible: Cable Barriers

Cable barriers use high-tension steel cables mounted on weak posts designed to break away on impact. The cables stretch and deflect during a crash, absorbing energy gradually and reducing forces on occupants. That deflection also means cable barriers need more room behind them, which suits wide medians on divided highways. AASHTO’s Roadside Design Guide recommends median barriers on high-speed, fully access-controlled roads where the median is 30 feet wide or less and average daily traffic exceeds 20,000 vehicles.⁷

Rigid: Concrete Barriers

Concrete barriers, including the New Jersey shape and F-shape profile, deflect very little on impact and contain vehicles through mass and geometry. They are the right choice where there is almost no room behind the barrier, such as bridge edges, work zones, and narrow medians. The tradeoff is higher force on occupants, since the vehicle decelerates faster.

End Terminals and Crash Cushions

The exposed end of a guardrail is one of the most dangerous points on a highway if left unprotected. End terminals are engineered either to absorb the energy of a head-on impact or to guide the vehicle past the barrier end. Under MASH, non-gating terminals must contain and redirect any vehicle that strikes the nose, a stricter requirement than Report 350, which allowed vehicles to penetrate past some terminal designs.¹ Crash cushions serve a similar purpose at gore areas and fixed-object hazards, decelerating vehicles to a stop within tolerable force limits.

Older Hardware Already Installed

The switch from Report 350 to MASH happened on staggered deadlines. FHWA and AASHTO required MASH compliance for new W-beam barriers and cast-in-place concrete barriers by the end of 2017, for W-beam tangent terminals by mid-2018, for crash cushions by the end of 2018, and for all remaining devices (bridge rails, transitions, cable barriers, cable terminals, sign supports, and other breakaway hardware) by the end of 2019.⁸

Those deadlines apply to new permanent installations and full replacements on the National Highway System. Existing devices that passed Report 350 can stay in service as long as they remain in crashworthy condition, though FHWA urges agencies to phase out older hardware over time, especially devices that were never tested to Report 350 or later. Temporary work-zone devices manufactured before the end of 2019 and tested to Report 350 or the 2009 edition of MASH can continue in use through their normal service lives.

Who Installs and Maintains Guardrails

FHWA writes the standards and controls the money; state DOTs do the work. Under federal law, each state transportation department supervises construction on federal-aid highways and must follow both state and applicable federal requirements.⁹ For guardrails, that means site evaluation, hardware selection from FHWA’s eligible device list, installation, and ongoing inspection. Only new posts and rail elements with a material certification are allowed on federal-aid projects or National Highway System routes.¹

Federal regulation 23 CFR 625 incorporates the AASHTO design standards, including MASH, by reference into the requirements for all federal-aid highway projects.¹⁰ A state may delegate the physical work to county or local agencies, but accountability stays with the state DOT. If the Secretary of Transportation finds a federally funded project is not being properly maintained, the state is notified and expected to correct the problem.⁹

When a Guardrail Is No Longer Functional

A damaged guardrail that still looks roughly intact may not perform if hit again. FHWA’s repair guide considers a section non-functional if the rail element is torn or separated, if the top of the rail has dropped to 24 inches or below, if the rail is pushed 18 inches or more out of alignment regardless of post damage, or if three or more posts are broken, bent, or separated and the rail is pushed 12 inches or less out of alignment.¹¹ Any of those conditions means the repair needs priority scheduling. For end terminals, the rule is simpler: if the energy-absorbing head is off the rail, or if the terminal is partially destroyed and exposes a blunt end, it must be replaced.

Agencies should also consider upgrading an entire installation to the current standard when the damaged section exceeds roughly half the total length, or when the installation is short (100 to 200 feet) and the damage is severe enough to justify starting over.¹¹

Federal Funding for Guardrail Projects

Guardrail installation and upgrades qualify for federal funding through several programs under Title 23 of the U.S. Code. Federal highway law explicitly includes guardrails in the definition of “highway” and treats elimination of roadside hazards as an eligible construction cost.¹² The two most relevant programs are:

Highway Safety Improvement Program (23 USC 148), which funds projects that achieve a significant reduction in traffic fatalities and serious injuries on public roads. Installation of guardrails, barriers, and crash attenuators is specifically listed as an eligible project type.¹³
Hazard Elimination Program (23 USC 152), which requires each state to survey all public roads for hazardous locations and roadside obstacles, then prioritize improvements. The federal share for projects under this program is 90 percent of the cost, and states must report annually on progress and safety benefits.¹⁴

Upgrading older guardrails to MASH fits both programs, since replacing outdated hardware eliminates a roadside hazard and counts as a safety improvement.

Heavier Vehicles and What Comes Next

Electric vehicles present a new challenge. Battery packs add substantial weight, and many EVs weigh significantly more than their gasoline equivalents. AASHTO acknowledged in a 2023 joint position statement that EV battery weight causes more wear on roads than they were designed for and that safety and design standards need to account for emerging technologies. A 2025 AASHTO policy resolution noted that states face challenges investing in safety countermeasures as vehicle designs continue to change.

The current edition of MASH (2016) does not include EV-specific test vehicles. AASHTO has said future editions should include updated test matrices, research on critical impact points and angles, and adjustments based on in-service performance. No formal revision adding heavier EV test vehicles has been published yet. In the meantime, the 31-inch MGS rail height and the heavier test vehicles MASH already uses give some margin over the older 27-inch designs, because the higher mounting engages the bumper and body structure of taller, heavier vehicles.¹

If You Hit a Guardrail

Drivers who crash into a guardrail can expect a bill from the state DOT for the repair. Most states treat guardrails as state property and pursue reimbursement from the at-fault driver or their auto insurance carrier under the property damage liability portion of the policy. Bills typically include labor, equipment, materials, and traffic control. Repair costs for even a short section of W-beam guardrail can run into the low thousands of dollars, and replacing an end terminal or crash cushion costs considerably more. Forward any such bill to the insurance company that covered you at the time of the accident, since property damage liability coverage generally applies. Some states send these bills months or even years after the incident, so a delayed bill is not unusual.

1
Federal Highway Administration. FAQs: Barriers, Terminals, Transitions, Attenuators, and Bridge Railings
2
Federal Highway Administration. About FHWA
3
Federal Register. Manual for Assessing Safety Hardware (MASH) Transition
4
Federal Highway Administration. Safety Eligibility Letter B-225
5
Federal Highway Administration. Hardware Eligibility Letters
6
Federal Highway Administration. Federal-Aid Reimbursement Eligibility Process
7
Federal Highway Administration. Median Barriers
8
Federal Highway Administration. Roadside Hardware Policy Memoranda and Guidance
9
Office of the Law Revision Counsel. 23 USC Ch. 1 Federal-Aid Highways – Section 106
10
eCFR. 23 CFR 625.4 Standards, Policies, and Standard Specifications
11
Federal Highway Administration. W-Beam Guardrail Repair Guide for Highway and Street
12
Office of the Law Revision Counsel. 23 USC Ch. 1 Federal-Aid Highways
13
Office of the Law Revision Counsel. 23 USC 148 Highway Safety Improvement Program
14
Office of the Law Revision Counsel. 23 USC 152 Hazard Elimination Program