The universally known traffic light has not experienced a significant redesign in almost a century, ever since Detroit police officer William Potts created the first three-section traffic light in the United States in 1921. Now, experts suggest that the rise of autonomous vehicles necessitates new safety guidelines to ensure they interact correctly with traffic signals.

Globally, traffic lights use red, amber, and green lights to instruct drivers when to stop, proceed, or prepare to do either. Ali Hajbabaie, an engineering professor at North Carolina State University (NCSU), is spearheading the design of a traffic system that factors in how self-driving cars respond to traffic signals.

Hajbabaie told The Associated Press that they propose adding another light—potentially white.

Traffic signals are about more than just safety; they enhance traffic flow, reduce congestion, and economically benefit society by saving time and fuel.

The 2022 Global Traffic Scorecard by INRIX found that road congestion in the United Kingdom, Germany, and the US cost the economy $2.2bn. Traffic lights play a pivotal role in lowering this financial burden.

What additional insights can history provide for the future?

When was the traffic light invented?

The first-ever traffic light was installed on Parliament Square in front of the Houses of Parliament in London, UK, on December 10, 1868.

Growing congestion from horse-drawn carriages at that intersection posed increasing danger to pedestrians, prompting the installation.

The device featured a pillar with an upper section resembling a cross, using semaphore arms to signal whether to move or stop, and red or green gas lights.

Designed to imitate a traffic policeman's gestures, it required traffic to halt when both arms were perpendicular and to move when they were at a 45-degree angle, with red and green gas lamps used at night.

This system, based on railway signals, needed a policeman to operate it, using gas fed through a pipe to power the lights.

Unfortunately, the first design had a rocky start. A gas leak from the supply pipe caused an explosion in the mechanism, injuring the operating policeman.

Deemed hazardous, the system was rapidly removed, and traffic lights were banned for the next 60 years. They returned to British streets in 1929 after the invention of the three-light signaling system in Detroit in 1921.

The first electric traffic lights were invented in 1923 by Garrett Morgan, an African-American inventor, who sold his design to General Electric for $40,000 ($730,000 adjusted for inflation).

Why are traffic lights red, amber, and green?

The color system for early traffic signals was inspired by the nautical navigation lights on ships.

By using red and green lights, ship crews could instantly discern the direction of an approaching vessel, serving as an early collision-prevention system, especially in low visibility conditions.

The amber light was introduced in 1921 by William Potts, when creating the three-color traffic light in Detroit.

This added a yellow "caution" light to signal an impending change, advising motorists to slow down. The first four-way, three-color traffic light was installed at the intersection of Woodward Avenue and Fort Street in Detroit in 1921. By the mid-1930s, this standard became widespread across the country.

Why do we need a fourth color now?

Humans and self-driving cars interpret lighting systems differently. While flashing lights work best for humans, a single light works better for autonomous vehicles.

A fourth light, likely white, would be added for the benefit of self-driving cars, signaling "keep going unless instructed otherwise."

Professor Hajbabaie from NCSU explains: "If the white light is active, you just follow the vehicle in front of you."

What different types of traffic light systems are there?

Though various types of traffic signal technologies exist, most systems fall into two categories: fixed schedule signals and those that adjust timing based on traffic volume.

Different models include:

• Fixed-time traffic signals: These aren't based on traffic volume but set by planners to move traffic at predetermined times.
• Detection traffic signals: Known as "actuated traffic signals," these detect vehicle presence and volume at intersections to sync the traffic signal precisely.
• Split-cycle offset optimization technique (SCOOT): A traffic management system that optimizes traffic volume using real-time data and automatically adjusts light sequences based on collected data. This reduces congestion and improves flow.
• Pedestrian-activated traffic signals: Activated by a manual button or sensor that detects a pedestrian at the crossing and changes the lights to stop traffic.

Could AI be used in traffic systems?

Henry Liu, a civil engineering professor at the University of Michigan, is exploring a new approach to solving traffic congestion using artificial intelligence (AI).

Liu won a $15m grant last year from the US Department of Transportation to establish the Center for Connected and Automated Transportation. His team is testing how traffic lights could signal using real-time car location and speed data in Birmingham, a suburb of Detroit.

This neighborhood is ideal because all 34 traffic lights there operate on a fixed schedule and do not adjust for traffic flow using cameras or sensors. This makes it easier to adapt the system to car data without interference, Liu explained.