Generally, ATVs are not allowed on public highways, except to make crossings, and never on interstates or freeways. They can be ridden on city streets where cities have designated rights of way for ATV use, and along rights of way in counties where the board of supervisors has voted to allow such ATV riding. To drive on streets or county roads, you must have a valid driver's license and proof of insurance.
Check out these Garner, IA ATV/OHV resources: