Object lesson No 35 Traffic lights
In those days, all roads lead to Rome, but the build-up of traffic where they converged became so bad that Julius Caesar decided to ban wheeled vehicles from entering the city during daylight hours.
One-way roads and parking restrictions were introduced 300 years ago in several European cities, followed in 1868 in London by the first traffic lights.
These 22ft high hand-operated red and green semaphores, illuminated by gas lanterns at night, gave priority to MPs crossing Parliament Square on their way to vote. The following year they exploded, injuring a policeman and ending this experiment in traffic control systems.
Fifty years later, the first electrically operated lights, adapted from railroad signals, appeared on the streets of New York. But these simple stop-go indicators didn't stop collisions as horse-drawn carriages, automobiles and pedestrians competed for space.
After witnessing a particularly nasty accident, African American businessman Garrett Morgan patented a three-phase signal which was soon adapted to the red, amber and green version we seetoday.
The first automatic electric traffic lights in Britain were set up in Wolverhampton in the late 1920s. With no police presence to operate the signals or direct traffic, some drivers refused to obey the signals and the risk-taking, light-jumping "amber gambler" had arrived.
Traffic lights have changed a lot since then (about once every 40 seconds at busy city junctions) but their design is not much altered.
While these tricolour totems are part of the street furniture of Western cities, some countries, such as the Himalayan kingdom of Bhutan, still don't have any and traffic-clogged New Delhi installed computerised signals only this year. These vehicle-activated systems - which measure traffic flow through induction loops in the road and change signals accordingly - are behind most of the estimated 6,000 sets of lights in British cities.
Flower sellers and squeegee merchants count on traffic light queues for their custom, and atomic physicist Leo Szilard suddenly grasped the idea of nuclear fusion while waiting at traffic lights. Even journalists have good reason to be grateful to Mr Garrett Morgan, whose invention gave the green light to hundreds of cliched headlines.