The crash investigators were measuring the distance from the sign post to where its parts laid on the pavement. What you do is get that distance and use the calculation for how long it takes gravity to drop something from the height of the sign to ground.
And Speed = Distance the post traveled divided by the time.
For example, if the sign and post was 6 feet high, the time it took for it to fall and hit the ground would be .61 seconds. If the sign post landed 50 feet away, the speed of travel would be 82 feet per second or, 55.9 miles per hour. If the sign post landed 75 feet away, the speed of travel would be 84 mph (I used a feet/sec to mile/hour calculator). Hawthorne Boulevard has a speed limit of 45 mph. The air bags and seat belt saved him.
The air bags and seat belt saved him. No disagreement with your conclusion. However, your math assumes no initial vertical component(s) to the vector(s) of the sign part(s) immediately after the vehicle is no longer in contact with those parts.
Also, stricken parts would still have most of their horizontal velocity when initially hitting the pavement, and would to some degree slide, skip, tumble, or bounce.
No doubt corrections for all the above can be estimated by experienced investigators if they have time to do a thorough enough investigation. Angle of impact onto pavement seems like it could be "interesting" to try to determine.
I have great respect for what can happen: In one case, I witnessed the aftermath of a stricken pickup truck that was knocked by a slightly larger pickup truck over 100 ft., right in front of my house. The stricken truck had pulled out in front of the other and got t-boned. Path after impact = ~25 ft. of pavement, over a medium width ditch, across my front yard including a "bounce" @ the yard edge of the ditch, where the P.U. went airborne again, came down, took out a ~ 6" trunk dogwood tree, slid further -- just past the corner of my house, and impacted a roughly 14" dia. hickory tree with enough velocity to sink the hickory tree trunk it ended up "wrapped" on about a foot into the front end of the stricken pickup truck. The "striker" was slowed considerably by the impact but still veered slightly right across the intersection and about 75 deg. into the ditch "corner", coming to rest with the front end buried into the soft mud on the opposite side of that ditch. Amazingly, no one was seriously hurt. The speed limit at that spot is 55 mph, but the striker was not ticketed, only the "stricken" was, for pulling out into the path of the striker. (I suspect the striker was speeding, however.)
Actually, correction to my post: For some items, density, shape, airborne tumble or flutter, etc., would significantly affect air resistance and therefor also affect “flight behavior” / trajectory.