Let's suppose the speed limit on your street is 45 MPH. And you want your message to be readable for 3 full seconds by passing motorists. A car traveling at 45 MPH for 3 seconds equals a viewing distance of 198 feet. Again, the Lettering Sizing Calculator will automatically give me 2 sizes:
The minimum size the letters have to be to be readable
And
The size recommended for optimal visibility.
In this case, the minimum readable size is a 6" tall letter. And the size recommended for optimal visibility is a 20" tall letter.