I just received the "instructor's edition, specifically prepared for John Whitehead, Appalachian State U, compliments of N. Gregory Mankiw" of the 7th edition of "Economics". On page 221 it says:
Cost-benefit analysts have a tough job. Because the highway will be available to everyone free of charge, there is no price with which to judge the value of the highway. Simply asking people how much they would value the highway is not reliable: Quantifying benefits is difficult using the results from a questionnaire, and respondents have little incentive to tell the truth. Those who would use the highway have an incentive to exaggerate the benefit they receive to get the highway built. Those who would be harmed by the highway have an incentive to exaggerate the costs to them to prevent the highway from being built.