I was rear ended today, how fast was the other driver traveling?

My car is a Mercury Mariner the car that hit me as an Acura SUV. I was stopped in traffic and the impact came with a good back wards wallop, maybe because it was unexpected (I had my seat belt on, but I don't think it caught me). I didn't hurt initially, but I'm achy now, but not horrible.

My car's bumper is dislocated into the rear lights, which work, and off to the side. The reverse sensors were broken and the trailer hitch is pushed in, I think the hitch was the point of impact.

How fast was the other driver traveling? I'm not going to sue, I'm just curious.