Does Idaho have that law too? I grew up in CA and I remember when the speed limit was 55 for everything, and then they raised it to 65, but if you were towing a trailer, then it was still 55.
Every time I towed a trailer, I broke that law. I never got a ticket, but the cops would flash their headlights at me or pull up beside me and either honk their horn or turn on their interior light as they passed me. I always took that as a warning, so I slowed down. I don't remember anybody ever getting a speeding ticket for towing faster than 55, but I'm sure it happened.