How to Calculate Wind Loads From Wind Speeds | Sciencing talks about wind loading for 100mph of wind at around 50lbs per square foot. On a 10'x12' wall being a little over 6,000lbs. Similar ballpark size & force to a sign maybe. That's 3 tons of force, which is a lot in some contexts, but probably not this one.
Go to the car crash force calculator at
Car Crash - Impact Force Calculator - Omni & plug in 60mph & 10 tons & you get a resulting force of 18,340,645lbs or almost 10,000 tons. Not sure how accurate that is, but the force of a 10 ton truck at freeway speeds if going to be way more than 10 tons.
So even if the sign is a lot bigger & designed for more force, it's still going to be designed for a wind load less than a speeding truck. Although there is a pretty decent chance they do design the supports to not fall over & injure/kill innocent people when hit by a truck.