I think your WiFi idea is going to be the simplest and most reliable. 
Here’s the method I think most people are using now:
Another alternative, and the one I myself use, is an IBeacon, but that requires adding a couple of additional third party apps and works best if you have an iOS device.
I use the IBeacon method because I want the detection area to be very small, just the length of the wheelchair ramp to my front door. If I use a Wi-Fi message, it will detect me while I’m still on the bus at the end of the block and it can take the driver several minutes to get my wheelchair untethered so I can get off the bus. That would all be fine for your particular use case, I think, but it doesn’t work for mine because I don’t want things to start happening until I’m actually close to my front door.
If you’re interested in the IBeacon method, here’s the thread on that, but again I think the Wi-Fi message will be better for most people unless, like me, you want a very small detection zone.