What Causes Potholes?

Michael Pollick
Michael Pollick

Sometime around early spring, many roads develop deep divots and pockmarks called potholes, and certain cities are even said to have two seasons: winter and pothole repair. The reasons behind this road damage involves both nature and the limitations of road construction.

Cracks in asphalt allow moisture to seep into the underlying dirt and gravel.
Cracks in asphalt allow moisture to seep into the underlying dirt and gravel.

Most roadways are built in layers, starting with compacted earth and gravel for drainage. Some older city streets may even have a subsurface of bricks. All of these layers are covered with asphalt, which is a gooey blend of bitumen, oil byproducts, curatives and aggregate gravel. In an ideal setting, this layer of asphalt repels rainfall and snow, forcing it into drains or the shoulder of the road.

Potholes account for thousands of dollars worth of insurance claims per year.
Potholes account for thousands of dollars worth of insurance claims per year.

Potholes form because asphalt road surfaces eventually crack under the heat of the day and the constant stresses of traffic. These cracks allow snow and rainwater to seep into the underlying dirt and gravel. During cold nights, the water freezes and expands, pushing out some of the dirt and gravel, leaving a hole when the water eventually melts. Drivers continue to drive over these unseen holes, putting even more stress on the thin asphalt layer covering them.

Potholes can damage a vehicle.
Potholes can damage a vehicle.

Eventually, the asphalt layer over these divots collapses, leaving holes in the roadway. Potholes can cause significant damage to a car's suspension system or tires if the driver fails to avoid them. They can also fill with water, obscuring any other hazards they may contain. Even in places where the air temperature rarely falls below freezing, excessive rainfall or flooding can eat away at the road.

Excessive rainfall and flooding can damage roads.
Excessive rainfall and flooding can damage roads.

Road maintenance crews have two different way of fixing the damage. These repairs are roughly similar to a dentist using either a temporary or permanent filling material for cavities. During the winter months, potholes receive what is known as a cold winter mix. This is a temporary fix consisting of a soft asphalt poured into the holes after they have been cleared of debris. A layer of gravel may be added to increase strength and stability, but the damage is often expected to reappear by spring.

A more permanent fix is called a hot summer mix. This combination of roadgrade asphalt and aggregate is designed to last for years, but it can only be applied during dry, warm weather. When road crews use a hot summer mix, they often reroute traffic around the worksite and spend more time preparing the road surface for the patch. The finished layer of new asphalt is usually compacted to match the level of the road, making it nearly invisible.

Heavy traffic can wear down a roadway which eventually creates potholes.
Heavy traffic can wear down a roadway which eventually creates potholes.
Michael Pollick
Michael Pollick

A regular wiseGEEK contributor, Michael enjoys doing research in order to satisfy his wide-ranging curiosity about a variety of arcane topics. Before becoming a professional writer, Michael worked as an English tutor, poet, voice-over artist, and DJ.

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Discussion Comments


In regards to post 1 by "stepmasta," enough has been said about horses in the comments, so I won't bother to elaborate what others have said.

I would like to talk about "road tax." Road tax is not money that is paid to use the roads or even repair them, and is, in fact, a tax on vehicle CO2 emissions. Road maintenance is paid for by general taxation.

I believe this is fair, since whether you cycle, drive, walk or take the bus, your taxes go towards the roads. So if you ever read this comment, you can tell the guy down the pub that he's wrong.


The main cause of potholes is embrittlement of the pavement by low temperatures and/or UV light degrading the solvent fractions of the asphalt and/or water degrading the support of the ground below. So basically, unless in a cool dry place, you've got to keep working on it to keep it in good condition; crack sealing, surface dressing, resurfacing, good drainage.


What could be the main contributing factor to potholes in the tropics? There is no freezing weather so how does the road expand?


There are potholes in India too, where the temperature is always warm, between 20 to 50 degrees Centigrade.

Common people feel it is due to substandard work of road making and repairing. Accumulation of water is also felt to be a cause.

In some areas the road is topped with paving blocks instead of asphalt, which are tiles of interlocking shapes. These seem to have no tendency for potholes. However it is probably more costly.


I really can't deal with potholes anymore. They are costing me too much money.


They are very annoying. I've busted my tire because of potholes. It is horrible. I have to pay a lot of money to fix my car.


In regard to horses - it's been demonstrated in several communities (Amish) where there is a lot of horse traffic, that the horses can, in fact, contribute greatly to the pothole problem. The reason is due to the use of studded horseshoes. This puts all of the weight of the horse on just a few points on the pavement. In other words, I can drive over a log and it'll survive the weight of my car, but I can split the log using far less force, as long as I'm using a wedge to concentrate that force over a smaller surface area.

The cracks caused by the horseshoes don't necessarily create potholes; the main agent responsible is the freeze/thaw cycle with water. But, without cracks in the road, the crown of the road causes most of the water to simply run off.


Potholes are primarily caused by improper compaction during the road construction, diesel and scratches as a result of accident and also fire.


Could potholes be caused by snow plows? What about a combination of the water freezing raising the asphalt and then the plow making the hole. The question I am asking is when the hole is made, are all pieces of the road accounted for?


I'm impressed by the very informed comments and opinions made on potholes.

I have one concern I'd appreciate your comments on: would sand and stones (especially) cause or contribute to increased potholes on the roads?

It could be due to friction or any other factors I may not be thinking of. Thanks, Ian.


I don't believe that horses are the main cause of potholes on are roads. As i understand it the main cause of potholes are a combination of weather and diesel(petrol) spills. Diesel breaks down the bitumen that holds the stone together. That area becomes weak and when traffic passes over it, it breaks up the surface and causes a pothole.


That is the most ridiculous notion I've ever heard. Horses are not nearly large enough or heavy enough to cause potholes. Anyone who has thought about potholes, done a little research or examined the size of a pothole would know that potholes are not caused by horses. Instead, they caused by the expansion of water as it freezes in between cracks in the road.


a very annoying guy in my local pub was ranting on about how i shouldn't be allowed to ride my horse on the road. one reason being because i don't pay road tax for the horse. but he also said that it is a proven fact that horses are the main cause of potholes (and most of the other men at the bar agreed). is this true? i've read the pothole article on this site, and i see no mention of horses.

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