Every Program a guide drives with GPS tracking records the fuel it burned. Trailhead estimates the fuel each guide has left (the coupon liters they used minus fuel burned on completed Programs) and warns the guide before they run out, with the next step: use a coupon, request one, or wait for a request already sent.
Track
Fuel coupons
Phase
Track and warn
Starts with
Program drive completed
Ends with
Guide knows their estimated fuel and next step