Thought of this one a while ago. Some of my thoughts:

- How will you know where I am? It would be expensive to run my phone's GPS continuously to figure this out.

- My car might not be immediately available. The time it takes to get to the car will have little or nothing to do with my GPS distance from it.

- Parking can take a significant amount of time.

- Traffic estimates are often wrong, sometimes greatly underestimating travel time. Being late is much worse than being early, but this is appointment dependent.

Looking forward to more details!