TABLE OF CONTENTS To motivate the notion of requirements risk, here is a vignette. [1] On a road that the author travels, he passes a strange street sign that declares "End brake retarder prohibition." The meaning of this sign is hard to understand, and is blurred by the curious use of a quadruple negative (each word in the directive has the connotation of stopping something). As it turns out, this sign is an exquisite example of a "shall not" requirement as well as illustrating a poor requirements specification, namely, one that is ambiguous, vague, contradictory, incomplete, or contains a mixed level of abstraction.
What is a "brake retarder"? Briefly, it is a device used on large trucks to slow the engine down by allowing air to be exhausted out of the pistons, thus slowing the vehicle. Brake retarders are very noisy, and, as a result, many municipalities prohibit their use within city limits. By Pennsylvania law, appropriate signs must be posted with the instructions "Brake Retarders Prohibited Within Municipal Limits." The complementary sign then reads "End Brake Retarder Prohibition." The latter phrasing is, apparently, unique to the Commonwealth of Pennsylvania (O'Neil 2004). In some states the sign pair reads "No Jake Brakes" and "End Jake Brake Prohibition," the term "Jake" being a nick-name of a company that manufacturers the device, Jacobs Vehicle Systems.
Taking the Pennsylvania form of the sign, if we substitute appropriate synonyms:
end ? stop, brake ? stop, retarder ? stopper,...
