No app required.
That being said, this is kind of a non-issue already, I have heard a story about someone abusing the preempt sensors like 10 years ago and never since then. Maybe there already exists an encoding scheme.
IMO a huge majority of people realize how illegal it would be to mess with these things, and the risk/reward is very low. Anyone who would take that risk would probably just run a red light instead.
https://web.archive.org/web/20060317013200/http://www.i-hack...
Of course this article came with many disclaimers that to actually do this would be very illegal.
I was more than a little sad when I typed i-hacked.com into the browser and got redirected to some etsy page. Thank goodness for the Wayback Machine, I guess.
That is probably more useful for planning an attack on emergency vehicles.
Although why even bother with servers? Just call in a fake emergency (or cause a real emergency) at a place of your choosing, and the government will send emergency vehicles to you.