What is crash detection?

How crash detection works

Modern phones and watches carry an accelerometer (which measures sudden changes in speed and direction) and a gyroscope (which measures rotation). A crash produces a distinctive burst in those readings: a spike far above normal movement, often followed by a tumble and then very little motion.

Crash detection software watches for that pattern. Some products add other clues. Google says Pixel's Car Crash Detection (on Pixel 4a and later) can use the phone's location, motion sensors and nearby sounds. Apple describes Crash Detection on iPhone 14 and later and recent Apple Watch models as designed for severe car crashes: front, side and rear-end collisions and rollovers in passenger vehicles.

When the pattern matches, almost every system follows the same shape: an alarm or vibration, an "Are you OK?" prompt, a countdown, and then an action if nobody answers. The action is where products differ most.

Who gets contacted, and how

Some built-in phone features call emergency services when the countdown ends. Apple says that after a severe car crash, iPhone or Apple Watch sounds an alarm and shows an alert for 10 seconds, then calls emergency services after a 30 second countdown if you cannot respond, and messages your emergency contacts with your location. Google says Pixel asks if you need help, gives you 60 seconds to respond, and can then call emergency services for you.

Other tools, including many bike computers and safety apps, alert personal contacts instead: a message or notification to a partner, parent or friend with your location, who then decides what to do.

Holler is in the second group. Holler's crash detection is a beta, opt-in feature: a possible hard fall or crash followed by stillness starts an "Are you OK?" countdown, and if you do not respond, your Guardians are alerted with your location by email and push. Holler never contacts 911 or emergency services, so the person who gets the alert is the one who calls if needed.

Here is how the contact route plays out. You go down hard on a descent, the phone registers the impact and then stillness, and the countdown starts. You are dazed and do not tap it. Your Guardian gets an alert with your location, opens the live link, sees your dot stopped on the road, tries calling you, and calls 911 with that location when you do not answer. Our answer on getting help if you get hurt while cycling alone covers what you can do yourself in those first minutes.

Limits and failure modes

No crash detection catches every crash. Apple states plainly that Crash Detection cannot detect all car crashes, and Google says Pixel may not detect all crashes and that high-impact activities could trigger it by mistake.

The miss cases matter most for people on foot or on a bike. A slow slide on gravel, a trip on a curb, or a fall where the phone is in a bag rather than on your body may not produce the violent spike the software is looking for. A car-focused system tuned for highway speeds may not be designed for a cyclist at all, which our answer on whether iPhone crash detection works while running or cycling explores.

False alarms go the other way: dropping a phone on concrete, jumping off a curb on a mountain bike, or a hard stop can all look like an impact. That is why the countdown exists. Keep your phone where you can reach it and cancel quickly.

Holler's beta has its own specific limits. It can miss real crashes and can false-alarm. In a browser, it only works while the page is open and the screen is on, so a phone that locks in your pocket will not be watching.

Crash detection compared with fall detection and stationary alerts

Fall detection is closely related but tuned for a person falling, typically from standing or walking, often on a wrist-worn device. Crash detection usually refers to vehicles or bikes at speed. The sensors are similar; the patterns they look for are not.

A stationary alert ignores impacts entirely. It only asks: have you stopped moving for longer than you expected? It is slower to fire than crash detection but catches situations crash detection cannot, such as a medical problem or a fall with no dramatic impact. Many people run both, because each covers the other's blind spots.

A manual SOS is the version where you do the detecting. It is the most reliable trigger of all when you are conscious and can reach your phone.

Setting it up so it can help

Cyclists weighing options can read our guide to crash detection options for cyclists, and anyone who has just gone down should read what to do after a bike crash.