The landscape architect and engineer in charge of the newly named Garden State Parkway was Gilmore David Clarke of the engineering firm Parsons Brinckerhoff, who had worked with Robert Moses on the parkway systems around New York City. Clarke's design prototypes for the parkway combined the example of the Pennsylvania Turnpike, a model of efficiency with parallels in the German Autobahn routes of the 1930s, with the Merritt Parkway model that stressed a planted "green belt" for beauty. Both design models featured wide planted medians to prevent head-on collisions and mask the glare of oncoming headlights. The Garden State Parkway was designed to have a natural feel. Many trees were planted, and the only signs were those for exits—there were no distracting billboards. Most of the signs were constructed from wood, or a dark-brown metal, instead of the chrome bars used on most other highways. The guardrails were also made from wood and dark metal. Most early overpasses were stone, but were later changed to concrete, with green rails and retro etchings, popular around the 1950s and 1960s. The parkway was designed to curve gently throughout its length so that drivers would remain alert and not fall asleep at the wheel.