I have a personal radar set up on my home PC that is scanning spawn points within drivable distance from my house. If something that I want spawns, I get an alert.
Alternatively, people in some cities have set up automatic twitter feeds for rare pokemon so you can get notifications on your phone. For instance, downtown LA:
https://twitter.com/PokeDTLA
You can google reddit + pokemon go + your location to see if there is something similar in your area. Of course, you have to less than 15 minutes to drive to the spawns. But please don't look at the phone while your car is moving. No pokemon is worth an accident