How much to tip shuttle services?

[Free4Life11]

This isn't Disney related, but I am taking a shuttle service to the airport, about 55 miles away. It is $65 roundtrip, but I am wondering how much I should tip each way. Thanks! Also, is there a general % for tipping cabs? I will be using a few cabs in Chicago.

 

[goofy4tink]

I usually tip at between 15-20% for my shuttle rides.