Federal Highway Administration, U.S. Dept. of Trans., Manual on Uniform Traffic Control Devices 1A-1 (1988). The manual also states that, in order to be effective, traffic signs should meet the following basic requirements: (1) fulfill a need; (2) command the attention and respect of operators; (3) convey a clear, simple meaning; (4) give operators adequate time for response. See id. Furthermore, with regard to warning signs, the manual dictates that such signs "should provide adequate time for the driver to perceive, identify, decide, and perform any necessary maneuver." Id. 2C-3.