The rainfall simulator at the Utah Water Research Laboratory, Utah State University, Logan, Utah, was utilized in evaluating various types of license plates. This unique facility enables the production of controlled uniform rainfall inside the laboratory. For this experiment, rainfall rates of 0" (dry), 1/2" and 2" per hour were used to compare reflective performance of reflective sheeting (RS), beads-on-paint (BOP), and new paint (NP) license plates under dark nighttime conditions. Both new and used license plates were evaluted. The three types of new plates were made and embossed at the Idaho State Prison using stanard state manufacturing procedures. The used plates were from the current Idaho issue (RS) and from Missouri (BOP). The used plates averaged 1 year exposure for the BOP plates and over 3 years for the RS plates. Reflectance of plates was measured under various wet and dry nighttime conditions using appropriate photometric equipment. Legibility of the plates under these same conditions as well as in daylight-dry conditions was determined by using selected trained human observers. Results of both the photometric and legibility parts of these tests indicate the superiority of RS plates over both the BOP and NP plates.
Israelsen, C. Earl and Canfield, Ronald V., "Evaluation of Legibility of Reflectorized License Plates" (1980). Reports. Paper 374.