A new Michigan law that doubles the jail sentences and license suspensions for first-time offenders who drive after drinking excessively is intended to keep "super-drunk’’ motorists off the state’s roadways, a state lawmaker says. One provision of the law that took effect Sunday essentially creates a new class of blood-alcohol content of 0.17 percent or higher for serious first-time offenders. That’s more than twice the minimum of 0.08 percent now required for a drunken driving conviction.