Message viewing time is the length of time that the message is potentially readable by a motorist.  Knowing how far away the sign becomes legible and how close to the sign it becomes illegible, and knowing the speed of the motorist, we can compute the maximum viewing time. 

Drivers must have enough time to read a sign message twice before passing under the sign location.  The second reading allows drivers to confirm information (i.e. the incident location or direction of travel affected).