Federal agencies are running late in starting a congressionally mandated nationwide test that taxes motorists for the miles they drive instead of the gallons of fuel they buy, something most states have already started exploring. For the national effort to prove helpful, transportation experts say, it should remain simple.

The goal should be to “only test what absolutely needs to be tested at the federal level” for imposing mileage fees on passenger vehicles, wrote experts from the Eno Center for Transportation, a nonpartisan think tank in Washington, D.C., in a recent report. They pointed out that dozens of states have already explored different ways of rolling out mileage taxes.

Federal officials should also avoid using overly complicated rates that would, for example, reward people for using fuel efficient vehicles or buying smaller vehicles, they cautioned.