the rolling straight-edge as a quicker method. The original design only had wheels at either end of the straight edge but this proved liable to error from small bumps so additional wheels were added. The current British practice as set out in its national standards, the Specification for Highway Works, is for the rolling straight-edge to be used for checking surface regularity of all sections of road longer than in length. The manual graduated wedge method is retained for use on shorter sections of road or where the rolling straight-edge proves impractical to use. The specification sets out the number of deviations greater than and greater than that are permitted per section of carriageway. The specification allows for two levels of tolerance, for category A roads and category B roads, with the client specifying which is to be used. Twice as many deviations are permitted on category B roads. No deviation greater than is permitted. In the United States rolling straight-edges have previously featured in some state highways specifications. Devices as long as and have also been used. The National Academies of Sciences, Engineering, and Medicine's journal Transportation Research Record in 1996 described the instruments as "cumbersome devices with limited production capability" and noted that they could miss deviations with a wavelength of half the length of the straight-edge, due to the fixed points of reference at either end. By 2001 36 US state departments of transport were specifying the use of profilographs to derive a profile index as a measure of surface regularity, rather than rolling straight-edges. The profilograph takes a series of laser measurements of the road surface along a defined track. The Taiwan Area National Expressway Engineering Bureau specification requires a rolling straight-edge as a means of measuring surface regularity. It specifies a maximum of of deviation over any length, but contractors have criticised this as being unachievable. In Germany the equipment, in use since 1960, is known as a planograph and a straight-edge with length is used. References Pavement engineering Dimensional instruments