Human beings have been constructing roadways for over six thousand years. Early roadways carried pedestrians, hoofed animals, and simple wheeled vehicles such as wagons. As traffic volumes grew, it became apparent that traffic regulations would be necessary for safety and efficiency. In ancient Rome, Julius Caesar banned wheeled traffic within the city during certain hours of the day to alleviate congestion. But because most traffic moved at the slow walking speed of humans or animals, collisions between road users could be avoided through observation and negotiation without official rules to establish right of way. In areas of high traffic, roadways would sometimes become unsuitable for pedestrians due to congestion, mud or animal waste. This led to the construction of sidewalks along the sides of urban roads to provide better conditions for walking.
The invention of the bicycle in the nineteenth century allowed sustained travel speeds much higher than was possible on foot or by horse. Bicycles were convenient and less expensive and time consuming to maintain than horses. As bicycling grew very popular, bicyclists promoted the paving of smoother roads for faster and more efficient travel. Meanwhile, pedestrians and people traveling by horse power complained about collisions and near-misses with bicyclists traveling at high speeds, and called for bicycles to be banned. Bicyclists also collided with one another, especially at intersections, because they often did not see one another in time to stop. As traffic increased in the cities, travel by any mode became quite hazardous, but it would soon become worse. Near the turn of the century, horseless carriages constructed using bicycle technology and powered by steam, electric motor, or internal combustion engine began to appear on roads. These "automobiles," traveling even faster than bicycles, created much more severe injuries for those involved in a collision. In the early decades of the twentieth century, fatalities from automobile crashes in the cities reached epidemic proportions. Cities began to create traffic rules to reduce crashes, and traffic signals and police officers were used to direct right of way at major urban intersections.
One of the problems with the first traffic rules was that the rules were different from one city to another, or even from one street to the next. Sometimes road users would forget which rules applied in which city or on which street. Traffic signals meant different things in different places, but vehicle users would often travel between places with different rules. The resulting confusion would often create much worse crashes because each participant on a collision course would believe they had the right of way, and not slow down until it was too late. Some traffic rules were different from one type of vehicle to the next, but drivers would sometimes change between vehicles, and other drivers would often not know how to predict the actions of the driver of an unusual vehicle.
It quickly became apparent that uniform traffic laws would be needed across the country (and in fact across most of the world) so drivers could travel from place to place without needing to learn new rules. The rules would have to be simple enough for ordinary human beings to follow reliably, and would be designed to prevent, or at least minimize, crashes given the operational constraints of wheeled vehicles. All vehicle drivers would have to follow the same rules, regardless of vehicle type or maximum speed, in order for the system to work. The rules would also provide for efficient travel by making optimum use of the available roadway capacity. All drivers would need and want to cooperate by following these rules in order to travel safely and efficiently. Drivers who failed to cooperate could be punished by law.
A great deal of trial and error in those early years determined which rules worked, and which did not. Eventually the discipline of traffic science evolved to explain the abilities and limitations of human cognition and vehicle operation in traffic. It was determined that in order for vehicle drivers to avoid crashes, they needed to be able to see and predict those road users to whom they would be required to yield right of way. This information would need to come early enough for the driver to react appropriately. Because human drivers can see very well within a narrow field of view in front of them, but not well to the sides or behind, drivers of vehicles would have to yield to traffic already on the roadway ahead of them. Explicit right of way rules for junctions and lateral movements would be closely matched to the abilities of human drivers of wheeled vehicles, and head-turns would only be required in specific situations when drivers would know to expect traffic conflicts at either side. Over time, these rules were slowly and carefully updated using scientific analysis of crashes in order to minimize collisions between vehicles by addressing every potential conflict that anyone could think of.