I have done BTLE beacons before iBeacon protocol even existed. And again, they are not instant detection. On iPhones, 90% percentile detection occurs within 10 seconds, on android devices, its longer and very inconsistent between device manufacturers, and some, outright terrible. Even then there are other issues like device is monitoring too many iBeacons, then the delay is much longer. Other issues include temperamental things like if the user kills the app from task switcher, iOS prevents the app from waking up in background from things like beacon region entry (ibeacon detection) and other host of things that make it good enough for non-mission critical tasks.
Opening a car door while phone is in pocket is mission critical IMO. Working 95% of the time is not enough. The 5% will annoy the crap out of users.