Good point :-)
Let’s do some back of the envelope math.
Car is traveling 25 mph (11 m/s) on a residential street. The barcode is somehow encoded such that a Velodyne HDL-32E lidar can read it at the edge of its 100m range. I’m pinging google.com right now via my cellular connection and the response takes ~60 milliseconds. So in this scenario the car has ~9 seconds until it gets to the sign, and it gets data back in 0.06 seconds. That seems fine.
Car is traveling 65 mph (29 m/s) down a rural highway. The barcode isn’t legible to the camera until about 30m away.
wget tells me that the total time to fetch transportation.gov over my cellular data connection is 2.0s (for comparison, fetch time for google.com is 0.6s). Now the car has ~1 second until it reaches the stop sign and it takes 2 seconds just to fetch the data. That doesn’t seem fine.
So I guess further research is required :-)