In the early days of the automobiles, especially in the period where horses and "horseless carriages" shared the road, entering an intersection could be a precarious endeavor.
The predecessor of the modern timing-based three-colored traffic signal wasn't invented until the 1920s, meaning the first 25 years of safe automobile travel depended on police officers directing traffic or a variety of traffic control devices that varied town to town, state to state.
Below is an example from Burley, ID, featured in the new pictorial history book by the Times-News, Magic Valley Memories. The sign reads, "DRIVE CAREFULLY. Don't commit MANSLAUGHTER."