<P> Although the origin of the name is disputed, it is generally attributed to British M.P. James Callaghan who, in 1948, visited the country's Transport Research Laboratory which was working on a new idea for safe pedestrian crossings . On being shown a black and white design, Callaghan is said to have remarked that it resembled a zebra . </P> <P> Crossings were originally marked by beacons and parallel rows of studs . After isolated experiments, the zebra crossing was first used at 1000 sites in the United Kingdom in 1949 in its original form of alternating strips of blue and yellow, and a 1951 measure introduced them into law . In 1971, the Green Cross Code was introduced to teach children safer crossing habits, replacing the earlier "kerb drill". </P> <P> The lines of a zebra crossing are commonly laid down by a road marking machine . Because the width of crossing lines is wider than other traffic lines, the marking shoe of a zebra cross marking machine is accordingly wider . The machine is hand pushed . </P> <P> In the United Kingdom, lollipop men or women (School Crossing Patrols) frequently attend zebra crossings near schools, at the hours when schoolchildren arrive and leave . Their widely used nickname arose because of the warning sign they hold up as they stop traffic . It's a large round disc on a long pole and thus resembles a giant lollipop, although they were originally of a square design . </P>

What do the zig-zag lines mean in london