This happened to me once. It was a main road with heavy traffic. I pulled over to the right to let the ambulance pass, but because of traffic, I was effectively sitting on the right lane. Apparently the ambulance wanted to move to the right lane because they were gonna turn right at the corner up ahead. I felt like an asshole because to everyone else on the road, it looked like I intentionally blocked the ambulance. And the siren blaring right behind you while the driver is blasting the horn is very stressful.