There are so many problems with this.  Privacy, yes, is a big problem with tracking through a smartphone app.  But what about more practical questions like how to distinguish between someone who is a carpool passenger -- or even riding a bus or light rail?  The current system provides an incentive for those who choose more environmentally friendly alternatives, such as high mileage hybrids and electric cars -- is removing that incentive ultimately a social good?  The beauty of the gas tax is that its relatively simple to administer and eliminates the need for things like determining whether a car at the gas pump should or should not pay gas tax -- a process that will have to individually occur literally thousands of times each day.  The problem is that the state is not generating enough revenue through the current gas tax -- increase the gas tax if necessary, don't create a massive Rube Goldberg bureaucracy to do it.