U.S. states are facing a myriad of hurdles as they experiment with programs to eventually replace the motor fuel taxes that have paid for roads for more than a century. Those taxes are generating less each year due to inflation, fuel efficiency and the rise of electric cars. One popular proposal is to charge drivers by the mile rather than the gallon. But eight years after Oregon began a voluntary pilot program, it's still having trouble getting many people to sign on. Other ideas are to tax electricity from public vehicle charging stations or tack charges onto door-to-door package deliveries. States are now weighing whether to start making the programs mandatory.