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What is road spray

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Road spray is a dirty mist of contaminants in the air, thrown up by a vehicle driving over the road. Road spray is partly the cause for traffic film and can cause dirt and grime, even when the weather has been dry and there is no obvious origin for dirt on a vehicle.


Cause of road spray

Road spray is a mixture of different contaminants that are thrown up in the air when a vehicle moves. At slow speed, this fine mist is thrown up very high, but at motorways this can create a large cloud of contaminants that hang over the motorway. The mist consists out of very small contaminants like dirt, sand, dust, rubber particles, engine oil, iron particles, tar, insects and sooth. When a vehicle drives over the road, the air is disturbed creating a vortex behind the car. Due to the shape of a car, the air over the vehicle has to cover a larger distance then the air underneath a car. At the rear of the car, this causes a suction effect that is similar to the effects from an airplane wing. This area is called the slipstream. When the air underneath the car is lifted up, up sucks very light contaminants up in the air. These contaminants are not much bigger then specks of dust, but they are in extremely large numbers. When the next vehicle drivers through this mist, some of these particles hit the surface and might stick to it. Other parts of a vehicle that can have an effect on the air are wheels, spoilers, objects on the underside of the vehicle, mirrors and in some cases even the rear bumper (like in the case of having wind-deflectors).

When it rains, the spray lifted up by a vehicle is not only these dust particles, but also water with contaminants inside the rainwater. The water mixes with the dirt on the road, creating a very fine mist of water particles with many forms of contaminants trapped in it. When the heavy water droplets hit the surface of the vehicle behind the first car, they leave a wet and dirt spot on the surface. Because water holds the contaminants trapped, the effects of road spray is much worse when the road is wet.

How to avoid road spray

Avoiding road spray is very difficult. In dry weather, the dust will just leave a very light film of dust on the surface. The mist thrown up by the vehicles will linger for longer, because of its very low weight. Unless the wind carries it away, the mist can remain airborne for several minutes. When the road is wet, the contaminants are thrown up in the air together with the water. These water droplets are much heavier then just the dust particles, causing gravity to pull it down quicker. The wet road spray will only linger for tens of seconds. The length of the trail of road spray behind a vehicle will also be much less in wet conditions then in dry conditions. Avoiding this road spray can be done in 3 ways:

Driving far behind the vehicle in front

Wet conditions

If the vehicle in front of you is driving in wet conditions, and is travelling at 100 km/h, it travels 27.8 meters per second. Lets say the wet road spray lingers for 20 seconds, the minimum distance would be 27.8 * 20 = 556 meters.
If the vehicle travels at 40 km/h, it travels at 11 meters per second. The road spray will be lifted up less high and with less force, lets say it lingers for 5 seconds, the minimum distance would be 11 * 5 = 55 meters.
This shows that this trick is only reasonably doable when travelling at low speeds.

Dry conditions

The road spray will linger much longer in dry conditions. It might remain airborne for 2 minutes before completely gone. Driving st slower speed will cause a less large cloud of road spray. For the example below, we could estimate that the cloud would remain airborne for 30 seconds at 40 km/h. Looking at the travel distance at certain speeds as calculated above, the result would be:
100 km/h = 27.8 * 120 = 3336 meters (3.3 km)
40 km/h = 11 * 30 = 440 meters
The calculations above show that it would be very difficult to drive far enough behind another vehicle to avoid any road spray in dry conditions.

**It should be noted that the amount of dirt left behind on the surface is much greater in wet conditions. Driving 30 km through dry road spray will leave significantly less dirt on the surface then driving the same distance through road spray in wet conditions!

Driving in the other lane

Although this can only be done if there are more then one lane available, it can be annoying for other roadusers. Its effects are also very small, and might not be worth the trouble. Although the road spray will be largest right behind the vehicle, there will also be road spray from the wheels, which goes in several directions. The shape of the wheel can even force air to move into a certain direction. Driving on the other lane might make a difference, but it will not avoid it completely.

Drive slower

If possible, drive slower or use road with a lower maximum speed. When you drive slower, the mist thrown up in the air will be less high and linger for less long. This means that you can drive slightly closer to the vehicle in front of you, while catching less road spray. This also works when driving in another lane, because the spray to the sides will also be less large and linger for less long. It should be noted that you should not create dangerous situations by driving to slow on roads where low speeds can be dangerous.

Unavoidable area’s

It should be noted that is will be practically impossible to avoid any road spray on the underside of the vehicle, and the rear. The vortex created sucks the air up at the rear of the vehicle, basically blowing the road spray back at the rear of the vehicle. This is also one of the reasons why the back of stationwagons (“estate” in America) can get very dirty, very quickly. The only slight solutions for this would be to either use a wind-deflector in the rear bumper, or a spoiler. However, this might have an effect on the driving characteristics of the vehicle and even the fuel consumption.

Cleaning off road spray

Most of the contaminants from road spray are normal types of dirt that your auto-shampoo is designed for. A quality shampoo should be able to remove a very large part of the contaminants picked up by road spray. The quality of the protection also makes a large difference in how much the dirt will stick to the surface. A washmitt or microfiber sponge should be designed to lift up the dirt when coming into contact with it, removing it safely from the surface without being dragged over the surface. Due to certain particles (like iron, sand and insect) having a shape with several sharp corners, it is paramount that these are removed without moving them to much. A part of the road spray (mainly dust and sand) can be removed by spraying the vehicle with clean water, or even by using snowfoam. The added benefit of snowfoam, is that it will loosen up other dirt to make it easier to wash it off with a washmitt or similar.

Tips on making road spray less dangerous

Road spray can pose 2 types of dangers, apart from the road being slippery when wet:

  • Loss of view, either by making the windscreen wet and dirty but also because a wet road spray is a fine mist that prevents you from looking far ahead,
  • Affecting your view when wanting to use the wipers

In both cases the view can be negatively affected, which is a large cause for accidents on the road. In wet conditions, you can prevent part of the problem with a good glass sealant. This glass sealant will create a Hydrophobic layer on the surface, preventing the mist of water to warp your view. Using your wipers will throw these beads of water from the windscreen and you have a much clearer view through the glass with less warp. This however does nothing about the decreased vision when the fine road spray lingers in the air in front of you. In extreme cases, the road spray can limit your view to only tens of meters, which can create very dangerous conditions.
In dry conditions, you won’t have to worry about the warped view or the mist of water that limits your vision, because the road spray is easy to look through. In many cases, the road spray is hardly visible at all. However, the windscreen might slowly get dirty which sometimes lead to people turning on the windscreen wipers to wipe off what they believe is just dust. Unfortunately, the film over the glass is not just dust, but many other particles that created a large smear over the glass. Using your wipers will smear these over the glass. This smear can warp your vision and in some cases even makes it a bit more difficult to look through. If this happens without the driver expecting this, it might catch the driver by surprise, creating a situation in which the driver is not in total control. This can be potentially dangerous. The general advice is to always use a screenwash, or to pull over in a safe place and manually clean the window.

Damage from road spray

Although road spray is largely small parts of dirt, it can also contain small stones and larger objects. Especially in wet conditions it is very difficult to recognize these. This can cause stone chips and serious surface damage at the front of the vehicle. This type of damage often requires a respray. Another type of damage can come from chemicals on the road that might not be identified (and avoided) in time. In the winter there will also be salt on the road, creating a risk of rust due to the salt reacting with the metal parts from the car.

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