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If you’ve been logging any distance across Japan’s sprawling road network lately, you’ve probably experienced it: on a motorbike it feels super smooth and in a car, it’s the sudden silence. It turns out that quietness isn’t your imagination playing tricks on you, it’s a highly intended, award-winning piece of civil engineering.
On the bike, I first noticed it when the Shin-Tomei opened in Gotemba many years back. Going from the relatively rough Tomei to Shin-Tomei was relievingly smooth from the moment rubber rolled onto that darker section of new tarmac. Recently, on a semi-regular commute by car here in Yokohama on section of the Shutoko, the stark contrast in road noise got me looking into the pavement tech that is changing the acoustics of our local roads. Here is the secret behind Japan’s increase in whisper-quiet tarmac.
The Secret Under the Surface: Drainage Pavement
What we are increasingly experiencing on freshly paved surfaces, especially on expressways and urban arteries, is an advanced evolution of Porous Asphalt, known locally as Drainage Pavement (Haisui-sei hosou – 排水性舗装). While the acoustic benefits make for an incredibly smooth sensation, its primary purpose was initially safety during heavy rain. This porous asphalt has been evolving and the latest and greatest smoothest of the smooth here in Japan is referred to as Eco-Safe Pave.
The simplified illustration above helps us understand the magic of the recipe of that topcoat asphalt.
Traditional asphalt is a dense mixture of variously sized rocks, fine sand, and bitumen. Everything packs together tightly, leaving only about 3% to 5% air voids. This basically creates a hard, impermeable acoustic wall that sound waves bounce straight off.
Porous, and in this case quiet, asphalt utilizes an “open-graded” mixture. Engineers deliberately remove the fine sand and smaller crushed rocks, leaving mostly larger, uniformly sized stones bound together by highly durable, polymer-modified bitumen. This results in a strong but sponge-like structural matrix with 15% to 20% air voids.
Asphalt used to typically utilise a uniform stone aggregate around 20 millimetres and in the late 1980s the introduction of porous asphalt which now covers around 70% of major roads reduced it to 13mm. Then from 2015, NEXCO began rolling out 5mm aggregate with an even more advanced proprietary polymer bonding agent which is ultra thin and viscous on application, opening up of the pores between those smaller stones of the aggregate, yet its incredibly durable locking the stones in for the long run.
Defeating the “Air Pump” Effect
When hitting highway speeds, most of the roaring noise you hear from car tyres isn’t just rubber hitting the road, it is trapped air. As a tire rolls, the tread blocks compress against the pavement. The air trapped inside the tread grooves is squeezed under the vehicle’s weight, violently popping or hissing out the sides of the contact patch.
On an open-graded porous surface, that pressurized air has an escape route. Instead of violently escaping laterally to create noise, the air is pushed straight down into the empty voids in the asphalt. Those interconnected air pockets act as a massive array of acoustic dampeners, absorbing the sound energy rather than reflecting it back up.
Why haven’t I heard this on my bike? Between wind roaring around the helmet and the drone of the engine between your legs, we don’t hear this from the relatively tiny contact patches on our bike. Never the less, it was a driving force behind the evolution to Eco-Safe Pave.
The Original Baseline: How Quiet Was It?
Local adoption of this pavement kicked off aggressively when Japan’s expressway authorities (which later became NEXCO in 2005) began trial installations of porous asphalt in the late 1980s to reduce wet-weather traffic accidents. And it did with an 86% reduction in rainy weather accidents in the first year. But how much quieter was it?
When engineers first rolled out this original porous asphalt (which used relatively large crushed rocks around 13mm), early field tests in Japan demonstrated an average noise reduction of roughly 3 to 6 decibels compared to the standard, dense asphalt of the era. Because the decibel scale is logarithmic, a drop of just 3 dB effectively halves the acoustic sound energy. It was a massive leap forward for mitigating urban highway drone.
The Torrential Lifesaver
Because those deep voids are exceptional at absorbing air and sound, they are equally brilliant at absorbing water.
During our heavy typhoon and rainy seasons, rainwater enters the porous surface layer and is rapidly carried away through the pavement structure to roadside drainage. This reduces standing water and greatly lowers the risk of that unnerving hydroplaning. For all of us out there on two wheels, it dramatically cuts down on the blinding spray kicked up by the vehicles ahead of you too.
The Next Evolution: Eco-Safe Pave
The pavement spreading across our modern networks right now is actually a bleeding-edge evolution of that original technology. While the late-1980s porous asphalt was revolutionary, engineers went on to develope a vastly superior version technically called Small-Aggregate Porous Asphalt, officially branded as Eco-Safe Pave (エコセーフ舗装).
Instead of aggregate rocks around 13mm, the new Eco-Safe Pave typically uses an aggregate somewhere around 5mm. Because the stones are much smaller, the physical road surface is significantly flatter and smoother. By smoothing out the surface layer, Eco-Safe Pave reduces road noise by around an additional 6-8 dB compared to older porous asphalt. That is an astronomical reduction on top of the original 3 to 6 dB gains. And THERE is that super quiet surface that got me thinking and one of the major reasons it won a prestigious award from the Ministry of Land, Infrastructure and and Tourism in 2024.
The Grip Trade-off: What Riders Need to Know
So, how does it handle when you throw the bike into a corner? Eco-Safe Pave is I guess what you’d call an exercise in engineering trade-offs.
- Wet Grip: In the rain, this pavement is an absolute lifesaver. It actively drains water downward, allowing the tire tread to constantly bite into physical stone pushing the water down as well as aside rather than slipping and sliding on it, and drastically shortening braking distances.
- Dry Grip: Because Eco-Safe pavement surface is structurally 20% empty space, there is simply less physical road surface contacting the tire. The absolute maximum dry friction is marginally lower than on a perfectly smooth, dense tarmac. However, compared to the older chunky porous asphalt, the tightly packed smaller stones provide a much flatter, more continuous surface area, which would logically help claw back a significant amount of lost dry grip.
- Cornering Stability: Under heavy cornering, enormous lateral shear forces are transferred into the road surface. Eco-Safe utilizes that super sturdy polymer-modified binder that locks those relatively tiny aggregate stones tightly together. This creates immense structural rigidity. When you load up the suspension in a tight, high-speed corner, the surface matrix does not deform, translating to sharper, more stable feedback through the chassis as you crank up the demands on the rubber meeting the road.
Next time you are out carving through the mountains or navigating city arteries, pay attention to the tarmac. If it’s raining, pay attention to the spray from the vehicles in front. Is it less and is there less standing water? If you happen to be caged up in a car, what do you hear? Your senses aren’t messing with you as you are literally riding on the bleeding edge of global paving technology!
Short History or Porous Asphalt
It originated in the United States during the 1950s and 1960s with a dual focus on water management. Highway engineers developed “open-graded friction courses” to combat high-speed hydroplaning. By intentionally removing fine sands, these initial “popcorn mixes” allowed rainwater to drain directly through the surface layer, dramatically reducing windshield spray and glare. Simultaneously, environmental researchers pioneered full-depth porous pavements to allow water to naturally percolate into the ground, mitigating urban flooding, and replenishing local aquifers without requiring gutters and storm drains.
As the technology spread to Europe, engineers made a welcome discovery: the porous structure inherently mitigated traffic noise. By the 1980s, countries like Germany, France, and the Netherlands recognized that the interconnected air voids naturally absorbed the compressed air trapped beneath rolling tires. Consequently, Europe quickly pivoted the pavement’s primary use toward noise reduction on major motorways. However, early European adoption faced significant durability challenges, as standard bitumen struggled to prevent the fragile stone structures from breaking up or the pores becoming clogged under heavy traffic.
Recognizing the vast safety and acoustic benefits, Japan adopted the technology in the 1980s but sought to engineer a tougher version. To resolve the durability issues that plagued Western roads, Japanese engineers introduced the high-viscosity, polymer-modified bitumen mentioned above in 1989. This elastic, heavy-duty binder successfully reinforced the open stone skeleton, solving the critical flaw of loosing surface stones. The innovation was so effective that by 1998, Japan mandated porous asphalt as the standard surface for its expressways, ultimately leading to highly refined modern iterations like today’s 5mm Eco-safe pavements.
References:
Metropolitan Expressway Technology – Eco-Safe Pavement
https://www.shutoko.jp/ss/tech-shutoko/hairyo/3-4-5.html
Durability of Porous Asphalt
https://www.vejdirektoratet.dk/sites/default/files/publications/durability_of_porous_asphalt.pdf
General composition detailed by Nexco:
https://www.e-nexco.co.jp/data/9999/81893/81893502.pdf
Types and Characteristics of Asphalt Mixtures | Japan Asphalt Mixture Association
https://share.google/O0fBmdz87qXKA4HqI
History of polymer modified asphalt (PMA) for road pavement : Japan Modified Asphalt Association (JMAA)
https://www.jmaa.jp/e/history.html
Test labs.
https://www.pwri.go.jp/team/pavement/japanese/facility/noise_h15.pdf
Permeable pavements | KURU KURA
https://kurukura.jp/article/190611-10/
