1. A Loop-Track Experiment: Why Cars Driving at the Same Speed Still Caused a Jam
The experiment took place on a loop track with a circumference of 230 m (about 755 feet). 22 cars were lined up, and drivers were asked only to drive at about 30 km/h (about 19 mph) and keep roughly the same gap from the car ahead. [1] This speed was chosen for the experiment, not for highway driving.
On this loop, there were no other cars to crash into, no uphill slopes, and no merging traffic. However, after driving for a while, the cars got stuck in one spot, moving very slowly. The study states that when the density of cars passes a critical limit, small fluctuations persist and grow, creating a jam naturally. [1] Bottlenecks like construction or hills are just triggers, not the essential cause.
So, does this jam move in the same direction as the cars, or does it move differently?
2. Why the Traffic Jam Moves Backward, Against the Flow of Cars
In the experiment, the traffic jam moved backward, opposite to the direction the cars were driving. Its speed was about 20 km/h (about 12 mph), which is similar to the speed at which jams move backward on real highways. [1] Note that this speed is an estimate from the experiment and real-world measurements, so it may vary.
Inside the jam, cars nearly stop. When the front car starts moving, it leaves the jam, and another car joins the back. This cycle repeats, so the jam keeps its shape while moving backward. [1] This means that while cars move forward, the jam itself moves backward. This helps explain why the end of a jam seems to stretch backward.
But why does the jam grow backward if cars are not crashing into each other? [1]
3. How Drivers Slowing Down for Small Gaps Grows Into a Jam
The study explains that the cause of the jam is a simple habit: drivers slow down when the gap to the car in front gets too small. [1] This is normal driving to avoid a crash.
NEXCO East Japan’s traffic forecasters explain how this works on busy roads. If the first car slows down slightly, the car behind it has less space, so it brakes. The next car brakes too. This creates a "brake wave" that grows larger as it moves backward, eventually causing some cars to stop completely. [3]
How many cars are needed for this wave to persist? Data from real highways suggests that around 25 cars per km is the dividing line between free-flowing traffic and jammed traffic. [1] This is a rough estimate from observing one highway location for one month.
An article introducing Professor Nishinari’s research notes that 25 cars per km means a gap of about 40 m (about 131 feet) per car. [6] This is not a general standard for jams. If you divide 1000 m by 25 cars, you get 40 m per car. A car traveling at 100 km/h (about 62 mph) moves about 28 m (about 92 feet) per second, so 40 m is about 1.4 seconds of travel time. This calculation is based on the numbers in the article. [6]
Where do these small slowdowns start on Japanese highways?
4. How Speed Drops Unnoticed at Sags, Where Road Slopes Change
On Japanese highways, where do jams start? According to NEXCO East Japan, about 70–80% of jams are caused by traffic concentration, while 20–30% are due to accidents or construction. [2] Of the jams caused by traffic concentration, about 60% happen at uphill slopes and "sags," and about 10% at merging points. [2] These percentages are for traffic-concentration jams only, not all jams. The data comes from NEXCO East Japan, but the specific year and range are not stated in the column.
A "sag" is a dip where the road changes from downhill to uphill. The change in slope is very gentle, so drivers often do not notice their speed dropping. [4] When speed drops, the gap to the car in front shrinks, and the car behind brakes. If there is heavy traffic, this becomes a "brake wave" and turns into a jam.
In NEXCO East Japan’s 2023 Golden Week forecast, speed drops at sags were listed as a main cause for predicted jams longer than 20 km (about 12 miles). [5] Both the study and the sag data agree that small slowdowns grow in crowded roads. So, what can drivers do?
5. How Drivers Can Avoid Passing a Jam Backward to the Cars Behind
NEXCO East Japan asks drivers to watch for speed drops at uphill slopes and sags, not to close the gap too much, and to avoid frequent lane changes. [5]
Road authorities also try to help. In 2009, NEXCO companies used measures such as LED signs to encourage speed recovery at uphill slopes and sags. At three locations, traffic jam volume (length times time) decreased compared to days without the signs. [7] The reduction was 11.9% near Hanyu PA on the Tohoku Expressway, 4.4% near Sagamiko IC on the Chuo Expressway, and 2.9% near Hirokawa IC on the Kyushu Expressway. The jams did not disappear, but the volume decreased by less than 20%.
There is also a driving technique called "jam-absorbing driving," tested by Professor Nishinari’s lab and JAF. When approaching a jam, drivers should keep their speed down, maintain a large gap, and not worry if another car cuts in. [6] The goal is to absorb the effect of the car in front braking, so the wave does not pass to the car behind. The article notes that ant behavior was a hint for this method. [6] If many cars drive this way, the jam length might shorten, but if the jam is longer than 5 km (about 3 miles), it is hard to fix. [6]
One person cannot erase a jam, but everyone can help stop the wave from growing.
6. What Passengers Can Observe About Traffic on Their Next Highway Trip
When you ride on a highway next, look out the window at the cars ahead. The driver should focus on the road; observation is for passengers only.
Passengers can look for three things: Are brake lights turning on in a chain? Is speed dropping where the road changes from downhill to uphill? How do cars around you move as the gap gets smaller?
You can also calculate. 25 cars per km means 40 m (about 131 feet) per car. At 100 km/h (about 62 mph), that is about 1.4 seconds. At 80 km/h (about 50 mph), the speed is about 22 m (about 72 feet) per second, so the gap is about 1.8 seconds. [6]
If you want to read the original study, the 2008 paper by Sugiyama et al. is available for free on the Osaka University repository. [1] It is in English but includes photos and diagrams. NEXCO East Japan’s website also publishes traffic forecasts. [5]