NHTSA Safety Ratings Explained — How Stars Are Calculated
A complete explanation of the NHTSA 5-star safety rating system: how crash tests are conducted, what each rating measures, frontal vs. side impact tests, rollover resistance, and how to use ratings when comparing vehicles.
What Is the NHTSA Safety Rating System?
The National Highway Traffic Safety Administration (NHTSA) operates the New Car Assessment Program (NCAP), which evaluates the crashworthiness and safety features of new vehicles sold in the United States. Established in 1979, the program uses a familiar 5-star rating system to help consumers compare the relative safety of different vehicles.
NHTSA tests are conducted at the agency's Vehicle Research and Test Center in East Liberty, Ohio, using anthropomorphic test dummies (ATDs) equipped with sensors that measure forces on the head, neck, chest, and lower extremities during controlled crash events. The data collected from these sensors is used to estimate the probability of serious injury in real-world crashes.
It is important to understand that NHTSA star ratings compare vehicles within the same weight class. A 5-star subcompact car is not necessarily as safe as a 5-star full-size SUV in an actual collision between the two, because the larger, heavier vehicle has a physical advantage. The stars indicate how well a vehicle protects its occupants relative to other vehicles of similar size and weight.
The Three Crash Tests
Frontal Crash Test
The frontal crash test simulates a head-on collision between two vehicles of similar weight. The test vehicle is driven into a rigid barrier at 35 mph, with two dummies (a 50th percentile male and a 50th percentile female) positioned in the front seats. Sensors measure the forces experienced by each dummy's head, neck, and chest.
The 35 mph barrier impact is roughly equivalent to a 70 mph closing speed head-on collision between two vehicles of similar weight, making it a meaningful test of occupant protection. Ratings are given separately for the driver and front passenger.
Side Crash Test
The side impact test simulates a vehicle being struck on the driver's side at an intersection. A 3,015-pound moving barrier is driven into the stationary test vehicle at 38.5 mph. The barrier is shaped to represent the front end of a typical vehicle. Dummies are positioned in the front and rear driver-side seats.
Side impacts are particularly dangerous because there is less crumple zone to absorb energy compared to frontal crashes. Vehicles with good side ratings typically feature reinforced door structures, side curtain airbags, and torso-protecting side airbags.
Rollover Resistance
Unlike the frontal and side tests, rollover resistance is not assessed through an actual crash test. Instead, NHTSA uses a Static Stability Factor (SSF) calculated from the vehicle's track width and center of gravity height. Vehicles with a higher center of gravity relative to their track width (like tall SUVs) have a higher rollover propensity.
The SSF is supplemented by a dynamic "fishhook" maneuver test that evaluates whether the vehicle tips up during an aggressive lane-change scenario. The combined results produce the rollover resistance star rating, which ranges from 1 star (highest risk) to 5 stars (lowest risk).
What the Stars Mean
Each star rating corresponds to an estimated probability of serious injury in that type of crash:
| Rating | Estimated Serious Injury Risk |
|---|---|
| 5 Stars | 10% or less |
| 4 Stars | 11% to 20% |
| 3 Stars | 21% to 35% |
| 2 Stars | 36% to 45% |
| 1 Star | 46% or greater |
An overall safety rating is also assigned by NHTSA, but it is the individual test ratings that provide the most useful information when comparing specific crash scenarios. A vehicle might earn 5 stars in the frontal test but only 3 stars in rollover resistance, for example.
NHTSA vs. IIHS Ratings
The Insurance Institute for Highway Safety (IIHS) is a separate organization funded by insurance companies that conducts its own independent crash tests. The IIHS and NHTSA test programs are complementary, and you should consult both when evaluating a vehicle's safety.
Key differences between the two programs:
- Test methodology: NHTSA uses a full-width frontal barrier test, while IIHS tests include moderate overlap, small overlap (driver and passenger side), and an updated side impact test. The IIHS tests tend to be more challenging and reveal structural weaknesses that the NHTSA full-width test may not expose.
- Rating scale: NHTSA uses 1-5 stars. IIHS uses Good, Acceptable, Marginal, and Poor ratings for crashworthiness, and Superior, Advanced, and Basic for front crash prevention systems.
- Top Safety Pick: IIHS awards "Top Safety Pick" and "Top Safety Pick+" designations to vehicles that earn Good or Acceptable ratings across all crash tests and have adequate or better headlights. These designations are widely recognized as indicators of excellent overall safety.
- Coverage: NHTSA tests are taxpayer-funded and cover a broader range of vehicles. IIHS selects vehicles based on sales volume and consumer interest, so not every vehicle is tested by IIHS.
Using Ratings to Compare Vehicles
When using safety ratings to compare vehicles, keep these principles in mind:
- Compare within the same class. Stars are most meaningful when comparing vehicles of similar size. A 5-star midsize sedan and a 5-star midsize SUV are roughly comparable in occupant protection, but comparing a subcompact to a full-size truck is less useful.
- Look at individual test ratings. The overall rating can mask weaknesses in specific tests. Examine the frontal, side, and rollover ratings individually to understand the vehicle's safety profile.
- Check both NHTSA and IIHS. A vehicle that scores 5 stars from NHTSA may earn a Poor rating in the IIHS small overlap test. Consulting both sources gives you a more complete picture.
- Consider active safety features. Modern vehicles with automatic emergency braking, lane departure warning, blind spot monitoring, and adaptive cruise control have significantly lower real-world crash rates, regardless of their crashworthiness ratings.
- Weight matters. In a collision between two vehicles, the heavier vehicle has a significant advantage. If crash safety is your top priority, larger vehicles statistically offer better protection in multi-vehicle crashes.
Limitations of Safety Ratings
While safety ratings are valuable tools, they have important limitations:
- Ratings only evaluate new vehicles. As a vehicle ages, corrosion, worn components, and degraded safety systems can reduce crashworthiness below the original rating.
- Not every vehicle model and configuration is tested. The results for a specific trim or engine option may not perfectly represent a different configuration.
- Real-world crashes happen at infinite speeds, angles, and combinations that no standardized test can fully represent.
- Ratings do not account for crash avoidance technologies, which may prevent the crash from happening in the first place. Active safety features are increasingly important.