When we drove fleet owner trucks, two of the owners would pay our tolls if our in service numbers were 80% or higher. If you offer that bonus, be sure to be clear about the specified time period in which the bonus applies. In service numbers fluctuate week to week, month to month, and quarter to quarter.
A quarterly bonus tied to in service numbers seems reasonable. It allows the driver(s) flexibility to get home and time to keep the numbers above 80% by getting back out on the road. For safety reasons, I would recommend against bonusing in service time over 80%.
After we stopped driving fleet owner trucks and got into a truck of our own, our carrier started paying tolls or portions of them as part of the run pay. If you run trucks with that carrier (FedEx Custom Critical), that may muddy the toll bonus waters a bit. Still, a tolls-paid bonus is something to consider