signal tower was installed in Detroit. This traffic light using red-yellow-green was developed by William Potts, a Detroit police officer. Michigan is also home to the first snowplow. This winter maintenance started during World War I to keep of strategic highways clear. In 1919 Michigan first signed the trunklines, the second state after Wisconsin to do so. The first ferry service was started on July 1, 1923, linking Michigan's Upper and Lower peninsulas. The first gasoline tax was enacted in 1923 at the rate of $0.02/gal (equivalent to $/gal in ), but vetoed by Governor Alex Groesbeck. It was later enacted effective in 1926. The highway commissioner was also given complete control over the planning and maintenance of the state trunklines. Construction switched to concrete or asphalt only instead of gravel and macadam with an increase in the gas tax in 1927. Highway construction in the 1920s earned Michigan national attention. The first trunklline completed in concrete was M-16 (later part of US 16). The road was built to a standard of and between thick. The current standard at the time was wide and thick. The 1920s were also busy for Michigan highways as Michigan developed the yellow-line center line to indicate no-passing zones for sight-restricted hills and curves. Roadside picnic tables, soil testing and aerial surveying of highways also debuted at this time. As MDOT historians put it, "the age of mud was over; the age of concrete was moving in. Later history During the Great Depression, highway construction slowed down with decreased gas tax and property tax revenues. License plate fees were sent to the counties for road funding starting in 1932 and road crews made of "reliefers". The federal aid money was split between the highway department and the welfare department. The county welfare agencies supplied workers on road construction projects across the state. Roadside parks and travel information centers debuted in the 1930s as well. During World War II the department built the Willow Run Expressway and the Detroit Industrial Expressway in 11 months so workers could get to the Ford Motor Company's bomber plant at Willow Run. When the Interstate Highway System was created in the late 1950s, Michigan modified existing freeway plans to fit the Interstate standards. In the 1960s nearly of freeways were built at an average pace of one new mile every three to four days. Michigan was also the first state