More electric and fuel-efficient motor vehicles and a gas tax that hasnât been changed in 30 years are making it increasingly more difficult for Arizona to provide a highway system that can serve its rapidly rising population. During a recent briefing for the PHX East Valley Partnership, Floyd Roehrich Jr., an executive officer of the Arizona Department of Transportation, said shrinking revenue has cut by a third the stateâs annual spending on its highway system. As annual revenue has shrunk from $1.6 billion to about $1 billion, ADOT is focusing its dollars on preserving the stateâs highway network and cutting back on projects that upgrade or extend it.