Yes, Routes does offer unlimited mileage options on many of its rentals, but not all of them. Local renters aren't eligible for unlimited mileage rentals through Routes.
When offered, unlimited mileage is only for rentals within the state where the vehicle is rented. Your rental car is not allowed to leave the state of the rental location without written permission from Routes Car Rental (otherwise, an additional fee will be incurred).
If you plan to drive between states, other than a crossing fee, your mileage will be capped at 2500 miles per rental. Travel to Canada and Mexico is not permitted.
Please check your specific rental agreement for further information.
At Alternative Airlines, you can filter your rental search by mileage policy to find rentals with unlimited mileage. All you've got to do is make a search and select "unlimited mileage" from the filter menu on the search results page.