Which vehicles are not allowed to use a road sign-posted like this?

You got it right! This sign means that motorcycles are not allowed on the road where this sign is posted. It specifically applies to motorbikes (both with and without a sidecar). Cars and trucks are not affected by this sign and may use the road as usual. Always remember: a red circle with a motorcycle symbol inside means entry is forbidden for motorcycles only. This helps keep certain areas quieter, safer, or more suitable for other types of traffic.