Idaho Code § 40-815

The commissioners must each year, at the meeting at which they are required to levy the ad valorem tax for county purposes, estimate the probable amount of ad valorem tax for highway and bridge purposes which may be necessary for the ensuing year, and must regulate and fix the amount of ad valorem highway and bridge tax, and levy them. When all of the territory of a county is included in one or more highway districts the commissioners shall not regulate, fix or levy any tax for highway or bridge purposes.