What if cities had no traffic lights?

At a busy crossing, cars come from every side. Traffic lights help them take turns.

Red means stop. Green means go. What if there were no traffic lights at all?

Where two busy roads cross, cars, bikes and people on foot all want to go at once. Traffic lights solve that by taking turns: one direction gets green while the others wait at red.

But what if a city had no traffic lights at all? People would still need some way to share each crossing safely. In this thought experiment, Olly tries three ideas. Choose one to explore.

At an intersection, conflicting streams of traffic need a rule for who goes first, the right-of-way. Traffic signals assign it in timed phases.

This thought experiment removes signals and compares three ways of managing a crossing without them, or with better ones: right-of-way rules as at an all-way stop, the modern roundabout, and adaptive signal control. What happens across a whole city is reasoning, labeled as ideas; the safety figures come from published studies.

Traffic lights have three colors. Red means stop. Yellow means the light is about to turn red. Green means go.

People walking have their own lights too.

California's driver handbook sets out what each light means. Red means stop. Yellow means the light is about to turn red, so stop if you safely can. Green means go, but you must still stop for anyone already in the crossing. Walk and Don't Walk signals do the same job for people on foot.

Traffic lights are not new. The first in the world went up in London in 1868, outside the Houses of Parliament. A police officer worked its arms by day, and at night red and green gas lamps meant stop and go.

Drivers in California are told that a red signal means stop, yellow warns that red is coming, and green means go only when the intersection can be crossed safely; pedestrian signals tell people on foot when they may cross. Conventional signals run on preset daily timing plans.

Smithsonian Magazine describes the first traffic signal, installed on December 9, 1868, outside Parliament in Westminster. Designed by railway manager John Peake Knight, it copied railway semaphores: horizontal arms meant stop, lowered arms meant proceed with caution, and red and green gaslights did the job at night. A policeman still had to operate it by hand.

Imagine all the lights are off. Cars must stop and take turns, one by one.

That might be slow. A roundabout could help cars keep moving!

Switch the lights off and drivers need another rule. California's handbook says that when a traffic light isn't working, drivers must stop as if there were stop signs in every direction, then go when it is safe. Without signs, whoever arrives first goes first, and if two arrive together, the driver on the right goes first.

That works, but at a very busy crossing, everyone stopping and taking turns might make long lines. A roundabout is another answer: cars go one way around a circle and join when there is a gap.

Without signals, the default is right-of-way rules. California instructs drivers to treat a dark signal as an all-way stop; at uncontrolled intersections the first to arrive has the right-of-way, ties go to the vehicle on the right, and pedestrians always have the right-of-way.

It is reasoning, labeled as an idea, that a busy junction run this way might queue badly. The documented alternative is the modern roundabout: in the United States, traffic circulates counterclockwise around a raised center island and entering traffic yields to circulating traffic, at about 15 to 20 mph in urban settings (IIHS).

At a roundabout, everyone goes the same way around a circle. Cars have to go slowly.

So cars don't meet head on, and crossings are safer.

At an ordinary crossing, cars can meet at right angles, turn across each other or come at each other head on. At a roundabout, everyone travels the same way, slowly, around a circle. The Insurance Institute for Highway Safety says the tight circle makes drivers slow down, and the most serious kinds of crossing crashes become unlikely.

Studies of U.S. crossings changed from traffic lights or stop signs to roundabouts found 72 to 80 percent fewer crashes where people were hurt. People walking cross one direction of traffic at a time, over a short distance.

IIHS explains that at signalized or stop-controlled intersections, right-angle, left-turn and head-on collisions are among the most common and can be severe; at roundabouts these are essentially eliminated because vehicles travel in the same direction at low speed. US conversions from signals or stop signs showed 72–80% fewer injury crashes and 35–47% fewer crashes overall; European studies suggest pedestrian crashes fall by about 75% on average.

Roundabouts are not a fit everywhere: IIHS notes that space or terrain can rule them out, and that very unbalanced flows, or a single intersection within a network of signals, may suit them poorly.

Some new traffic lights are smart. Sensors notice the cars.

Then the lights change at better times, so cars wait less.

Instead of no lights, some cities use smarter ones. Ordinary lights follow the same timetable every day. Adaptive signals use sensors to measure the traffic, then adjust the timing of the lights to fit, updating every few minutes.

The U.S. Federal Highway Administration says that on average, adaptive signals cut travel times by more than 10 percent, and by 50 percent or more where the old timing was badly out of date. Modern roundabouts are spreading too: the first ones in the United States were built in Nevada in 1990.

FHWA describes adaptive signal control technology (ASCT) as collecting data from sensors, evaluating it and updating signal timing, repeating every few minutes, rather than relying on pre-programmed time-of-day plans. On average it improves travel time by more than 10%, and by 50% or more where timing was outdated; systems include SCOOT and SCATS.

IIHS records that the modern roundabout was developed in the United Kingdom in the 1960s, that the first US examples were built in Nevada in 1990, and that a roundabout's service life is about 25 years, against about 10 for a typical traffic signal.