To clarify.

1. There is an international driving treaty. It requires all signatories to allow foreign visitors to drive if they are a) from another country who is party to the treaty, and b) has a foreign license and and either a certified translation of their license or an IDP.

2. In Japan, they ignore the treaty. They will not accept a translation. They require an IDP, but on the other hand they will almost never ask to see the original license.

3. In the US, most states do not require an IDP or a a translation. All that is needed is a valid drivers license from another country (or State).

4. Most of the cameras/sensors (other than ones around toll booths and highway entrance/exits are for measuring traffic flow and congestion. There are also quite a few speed cameras.