Vehicle Safety Ratings: How NHTSA and IIHS Tests Are Scored

Two Organizations, Two Different Approaches

When you look up a vehicle's safety score, you'll typically encounter ratings from two sources: NHTSA (the National Highway Traffic Safety Administration) and IIHS (the Insurance Institute for Highway Safety). Understanding what each one actually tests — and how they score results — prevents you from drawing the wrong conclusions.

NHTSA is a federal agency, so its testing program is part of the U.S. government's regulatory framework. IIHS is a nonprofit organization funded by auto insurers. Neither replaces the other; they complement each other by covering different crash scenarios and safety technologies.

If you're also researching a used vehicle, pairing safety ratings with a vehicle history report adds useful context. See how to read a vehicle history report to understand what that data can and can't reveal.

How NHTSA Scores Vehicles

NHTSA runs its New Car Assessment Program (NCAP) and rates vehicles on a 1-to-5-star scale across three main categories:

• Frontal crash: A vehicle hits a full-width barrier head-on at 35 mph, simulating a collision between two similar-sized vehicles.
• Side crash: A moving barrier strikes the driver's side, representing a vehicle running a red light and hitting another broadside.
• Rollover resistance: This is not a crash test — it measures a vehicle's susceptibility to rolling over based on its center of gravity and track width, expressed as a star rating.

Each category receives its own star rating, and NHTSA combines them into an overall vehicle score. Five stars is the highest possible. A lower star count in one category can pull down the overall rating even if the car scores well elsewhere.

How IIHS Scores Vehicles

IIHS uses a four-level descriptive scale: Good, Acceptable, Marginal, and Poor. Their test battery is broader than NHTSA's and includes scenarios that federal testing doesn't cover:

• Small overlap front: Only 25% of the vehicle's front corner hits a rigid barrier at 40 mph — a common real-world crash type that full-width tests can miss.
• Moderate overlap front: The driver's side faces a 40% overlap barrier at 40 mph.
• Side (updated): A heavier barrier is used to better represent the growing proportion of SUVs and pickup trucks on the road.
• Roof strength: Measures how much the roof compresses in a rollover.
• Head restraints and seats: Evaluates whiplash protection in rear-end collisions.
• Headlights: Rated separately — poor headlights can prevent a vehicle from earning Top Safety Pick.
• Vehicle-to-pedestrian front crash prevention: Tests whether automatic emergency braking detects pedestrians and cyclists.

IIHS awards Top Safety Pick or Top Safety Pick+ to vehicles that meet thresholds across all these categories. A single Poor rating in any area can disqualify an otherwise strong performer.

Why the Same Car Can Score Differently on Each Scale

It's entirely possible for a vehicle to earn five NHTSA stars but fall short of an IIHS Top Safety Pick — and the reverse can happen too. The main reasons:

• Different crash geometries: NHTSA's full-width frontal test spreads impact across the entire bumper. IIHS's small overlap test concentrates force at the corner, where structure is weaker on many vehicles.
• Different barrier weights: IIHS updated its side-impact barrier in 2021 to be heavier, reflecting real-world crash partners. Some vehicles rated well under older tests performed worse under the revised protocol.
• Different supplemental criteria: IIHS factors headlight quality and pedestrian detection into its top designations. NHTSA doesn't weight those the same way in its overall star score.

Treat the two rating systems as complementary rather than competing. A car that performs well on both gives you more confidence than one that excels on only one scale.

How to Use These Ratings Practically

Safety ratings are one useful data point among several — not a guarantee of safety in every real-world scenario. Here's a practical approach:

1. Look up the specific model year at safercar.gov (NHTSA) and iihs.org (IIHS). Don't assume a prior year's score applies to a newer or older version.
2. Read the individual category scores, not just the overall rating. A vehicle might have four-star overall NHTSA performance but a three-star side-crash score worth noting.
3. Check whether the IIHS rating used the current test protocol. Ratings from before a protocol update may not reflect current standards.
4. Consider active safety features — automatic emergency braking, lane-keeping assist — separately. IIHS evaluates these, but they also vary widely in how well different systems actually perform.

Knowing how to read these scores puts you in a stronger position when researching any vehicle. Just as you'd decode a dashboard warning light before deciding whether to drive, understanding a safety rating before you act on it is simply good practice.

This article is for general informational purposes only. Vehicle safety ratings reflect controlled test conditions and should be considered alongside other research. Always consult official NHTSA and IIHS sources for the most current data on specific vehicles.