This is tough because while the "obvious" solution is a laser bouncing off a reflector at garage door height, unless you have an extended overhang to mount it to, or mount it on arms extending away from the garage (may look... funny), you won't get enough warning before impact.
The other thought I had was get a motion sensor with an alarm, mount it to the top of the garage opening facing out, and then put something underneath to block the sensor from seeing "down" (like, put a small piece of wood or sheet metal to blind it from anything that would be below its line of sight), but I'm not sure how well that'd work in practice, and if you're too far away it would still end up setting it off.
I also just had the thought of getting some sort of two-part proximity sensor - one piece goes on the garage and the other goes on the roof rack. If they get too close together, you get an alarm. Not sure if there is anything out there like that, though.
The other thought I had was get a motion sensor with an alarm, mount it to the top of the garage opening facing out, and then put something underneath to block the sensor from seeing "down" (like, put a small piece of wood or sheet metal to blind it from anything that would be below its line of sight), but I'm not sure how well that'd work in practice, and if you're too far away it would still end up setting it off.
I also just had the thought of getting some sort of two-part proximity sensor - one piece goes on the garage and the other goes on the roof rack. If they get too close together, you get an alarm. Not sure if there is anything out there like that, though.
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