Laboratory-accelerated weathering tests are used instead of outdoor weathering tests in specifications for products intended for long-term outdoor use because they provide fast results. These accelerated tests are used as a measure of acceptable weathering performance for a material. Acceptance of results from laboratory-accelerated tests in product specifications is based on two major assumptions: (a) materials that pass the requirements of the laboratory-accelerated test meet outdoor durability requirements, and (b) results from laboratory-accelerated tests are much more consistent than those from outdoor exposures. Numerous examples illustrate the errors that occur when these assumptions are wrong. Material with good outdoor durability can fail to meet the requirements of a specified laboratory-accelerated test, or material with poor outdoor durability will pass the requirements of the specified laboratory-accelerated test. Between-lab variability can cause a product to be rejected by a specifier although the same test run by the supplier produced acceptable results. For retroreflective sheeting, outdoor exposures are actually more consistent than the laboratory-accelerated exposure most commonly used in specifications. To assure minimum acceptable durability, 36-mo outdoor exposures are recommended for most retroreflective sheeting specifications. However, results from these exposure tests do not predict the ultimate service life of retroreflective sheetings.