There was no big event, they just appeared a few at a time: handpainted signs in the teens, standardized shapes and wording by the 1930s. By the 1950s, the "need" for regulatory signs became a self-reinforcing cycle: the reference works (notably the Manual on Uniform Traffic Control Devices and the AASHTO Green Book) for traffic engineers had recommendations for where signage was appropriate, and various lawsuits slowly established that as the baseline of proper engineering practice, so that a jurisdiction that didn't install signs everywhere they were described as appropriate might be found negligent. Well into the 1980s, many large cities didn't have stop signs at intersections of residential streets. Drivers were expected to slow and yield to the driver on their right. But new drivers, trained in suburban areas with few four-way intersections and ubiquitous signage, didn't know this practice and it became more and more dangerous.

The need for so many regulatory signs is hotly debated today among transportation engineers. There are probably more stop signs in a US city of 50,000 than in all of the United Kingdom, where a simple line across the roadway indicates which street has the right-of-way. But in a nation like the US where government practices are so much determined by lawsuits (rather than a national regulatory authority), it would be very difficult to reverse the process of adding more and more signs to cover every possibility.