Local governments, schools and public agencies are bracing for the impact of a 26-cent-per-gallon state excise tax on gasoline, which will contribute to $850 million or more in revenue for road and bridge projects across Georgia, that takes effect Wednesday. The initial increase over the previous tax is about 6 cents per gallon. But with gas prices lower, on average, than they have been in many years, some agencies are able to manage the immediate hit.