FIELD NOTE · SUPPLEMENTAL SET
Both surfaces lose it: what cool pavement and permeable paving measure out at
ISSUEDJULY 11, 2026DRAWN BY THE NIGHTLY SWEEP
Permeable paving at Greenfield City Hall, Wisconsin, March 2014. Photo by Aaron Volkening CC BY 2.0
Both of these surfaces are bought for a number. Both give the number back.
Cool pavement is a coating that reflects instead of absorbing. Permeable paving is a surface built to let water through instead of shedding it. A city buys either one for a performance it cannot get from ordinary asphalt, and in both cases the performance is highest on the day it goes down.
That is not a defect. It is the product.
Mean pavement albedo went from 0.08 to 0.26 on installation. Within one year it had fallen to 0.18. A 30 percent reduction, in twelve months, of the exact property the coating was purchased for.
The temperature results are worth reading closely. They are smaller than the sales case. Maximum neighborhood-averaged surface temperature came down by 5 degrees C. The smallest reduction, measured in the morning, was 0.9 degrees. Air temperature moved 0.20 degrees C at midday, plus or minus 0.06, and 0.19 in the evening.
Two tenths of a degree of air. The surface is doing what it says. The air above it is barely aware.
Note the morning figure too. Nine tenths of a degree, at the hour a street is coldest and the sun is lowest. A reflective surface only works while something is landing on it, so the benefit arrives at the hour of peak sun and disappears the rest of the time.
WHAT THE COOL PAVEMENT DEPLOYMENT MEASURED, DEGREES C PEAK SURFACE TEMPERATURE REDUCTION 5 MORNING SURFACE REDUCTION 0.9 AIR TEMPERATURE REDUCTION AT MIDDAY 0.20 PEDESTRIAN MEAN RADIANT TEMPERATURE INCREASE 4
A reflective surface does not make the energy disappear. It sends it back up. Work cited in the same study found that cool pavements increased the shortwave radiation load on a pedestrian standing directly over them by up to 168 watts per square metre, which corresponds to a mean radiant temperature rise of 4 degrees C.
So the road is cooler and the body standing on it is warmer. Both are true. They are not in tension either, because they measure different things. One is the temperature of a surface. The other is what a person absorbs standing above it.
A city buying cool pavement to help people walking is buying a surface for a property it does not have.
Water carries sediment. The sediment lodges in the pore spaces. The research on clogging finds little change through the first year, then a rapid decline, reaching roughly half the original infiltration rate by year two or three.
Half. That is a surface specified to a rate, installed to that rate, and delivering half of it before the landscaping is established.
Maintenance recovers some of it. A study across ten permeable pavement sites in the United States and Sweden tested eight techniques, from manual removal of the top 2 centimetres of fill through mechanical sweeping, regenerative-air sweeping, vacuum sweeping, hand vacuuming and high pressure washing, to milling. Suction sweepers beat mechanical sweepers. None of it was 100 percent effective at removing the clogging material.
The one that nearly worked was the most aggressive. Milling to a depth of 2.5 centimetres brought a 21-year-old porous asphalt pavement back to close to like-new infiltration.
Which means the fix for a clogged permeable pavement is to machine the top inch off it.
That is not maintenance. That is a milling machine, a crew, and traffic control, on a surface somebody bought to avoid a storm drain.
An ordinary asphalt road degrades in ways everybody has priced for decades. These two degrade out of the function they were specified for. The recovery is a recurring specialist operation. Recoating on one. Suction sweeping or milling on the other. That work is a maintenance contract rather than a capital project, and maintenance contracts are the first thing a public works budget defers.
A surface that needs a vacuum truck twice a year to keep working is a surface that stops working in the third year of a tight budget.
The blind spot, stated plainly, is that this is partly a design question, and design does not measure with a straight edge. Somebody chose these surfaces for reasons that include how a street looks and how a city presents itself. Those reasons are real even when they cannot be weighed. What can be weighed is that the albedo fell 30 percent in a year and the infiltration halved by year three.
The same neglect shows up under an ordinary sidewalk, where the slab makes the root that lifts it and nobody funds the response either. Filings scored nightly for paving contractors across the Los Angeles record show the resurfacing and the site work while the surface is still being chosen.
Water still finds the low point and the sun still lands on the street. The surface is the only part anybody changed.
Both of these surfaces are bought for a number. Both give the number back.
Cool pavement is a coating that reflects instead of absorbing. Permeable paving is a surface built to let water through instead of shedding it. A city buys either one for a performance it cannot get from ordinary asphalt, and in both cases the performance is highest on the day it goes down.
That is not a defect. It is the product.
What the measurement actually found
A neighborhood-scale deployment was measured east of Los Angeles, in Covina, across about 0.64 square kilometres.Mean pavement albedo went from 0.08 to 0.26 on installation. Within one year it had fallen to 0.18. A 30 percent reduction, in twelve months, of the exact property the coating was purchased for.
The temperature results are worth reading closely. They are smaller than the sales case. Maximum neighborhood-averaged surface temperature came down by 5 degrees C. The smallest reduction, measured in the morning, was 0.9 degrees. Air temperature moved 0.20 degrees C at midday, plus or minus 0.06, and 0.19 in the evening.
Two tenths of a degree of air. The surface is doing what it says. The air above it is barely aware.
Note the morning figure too. Nine tenths of a degree, at the hour a street is coldest and the sun is lowest. A reflective surface only works while something is landing on it, so the benefit arrives at the hour of peak sun and disappears the rest of the time.
WHAT THE COOL PAVEMENT DEPLOYMENT MEASURED, DEGREES C PEAK SURFACE TEMPERATURE REDUCTION 5 MORNING SURFACE REDUCTION 0.9 AIR TEMPERATURE REDUCTION AT MIDDAY 0.20 PEDESTRIAN MEAN RADIANT TEMPERATURE INCREASE 4
The person on it gets hotter
Then there is the finding that should decide the specification, and it is not a degradation. It is an inversion.A reflective surface does not make the energy disappear. It sends it back up. Work cited in the same study found that cool pavements increased the shortwave radiation load on a pedestrian standing directly over them by up to 168 watts per square metre, which corresponds to a mean radiant temperature rise of 4 degrees C.
So the road is cooler and the body standing on it is warmer. Both are true. They are not in tension either, because they measure different things. One is the temperature of a surface. The other is what a person absorbs standing above it.
A city buying cool pavement to help people walking is buying a surface for a property it does not have.
Permeable paving has the same curve
The other surface fails the same way, on a slower clock and for a more obvious reason.Water carries sediment. The sediment lodges in the pore spaces. The research on clogging finds little change through the first year, then a rapid decline, reaching roughly half the original infiltration rate by year two or three.
Half. That is a surface specified to a rate, installed to that rate, and delivering half of it before the landscaping is established.
Maintenance recovers some of it. A study across ten permeable pavement sites in the United States and Sweden tested eight techniques, from manual removal of the top 2 centimetres of fill through mechanical sweeping, regenerative-air sweeping, vacuum sweeping, hand vacuuming and high pressure washing, to milling. Suction sweepers beat mechanical sweepers. None of it was 100 percent effective at removing the clogging material.
The one that nearly worked was the most aggressive. Milling to a depth of 2.5 centimetres brought a 21-year-old porous asphalt pavement back to close to like-new infiltration.
Which means the fix for a clogged permeable pavement is to machine the top inch off it.
That is not maintenance. That is a milling machine, a crew, and traffic control, on a surface somebody bought to avoid a storm drain.
Who pays for year three
None of that is an argument against either surface. It is an argument about the budget line that follows them.An ordinary asphalt road degrades in ways everybody has priced for decades. These two degrade out of the function they were specified for. The recovery is a recurring specialist operation. Recoating on one. Suction sweeping or milling on the other. That work is a maintenance contract rather than a capital project, and maintenance contracts are the first thing a public works budget defers.
A surface that needs a vacuum truck twice a year to keep working is a surface that stops working in the third year of a tight budget.
The blind spot, stated plainly, is that this is partly a design question, and design does not measure with a straight edge. Somebody chose these surfaces for reasons that include how a street looks and how a city presents itself. Those reasons are real even when they cannot be weighed. What can be weighed is that the albedo fell 30 percent in a year and the infiltration halved by year three.
The same neglect shows up under an ordinary sidewalk, where the slab makes the root that lifts it and nobody funds the response either. Filings scored nightly for paving contractors across the Los Angeles record show the resurfacing and the site work while the surface is still being chosen.
Water still finds the low point and the sun still lands on the street. The surface is the only part anybody changed.