Hydroplaning or aquaplaning by a road vehicle occurs when a
layer of water builds between the rubber tires of the vehicle and
the road surface, leading to the loss of traction and thus
preventing the vehicle from responding to control inputs such
as steering, braking or accelerating. It becomes, in effect, an
unpowered and unsteered sled.
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Causes
Causes
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Every vehicle function that changes direction or speed, from
turning, to accelerating, to braking, places an increased load
on the tires. Control of this load relies on the friction between
the tires and the road surface. If water comes between the
tires and the road, friction may be reduced to the extent that
the tires may slip, and the driver may lose control.
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The grooves of a rubber tire are designed to disperse water
from beneath the tire, providing high friction with the road
surface even in wet conditions. Hydroplaning occurs when a
tire encounters more water than it can dissipate. Water
pressure in front of the wheel forces a wedge of water under
the leading edge of the tire, causing it to lift from the road.
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The tire then skates on a sheet of water with little, if any,
direct road contact, and loss of control results. If multiple
tires hydroplane, the vehicle may lose directional control
and slide until it either collides with an obstacle, or slows
enough that one or more tires contact the road again and
friction is regained.
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The risk of hydroplaning increases with the depth of
standing water and the sensitivity of a vehicle to that water
depth.
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Factors that affect water depth include:
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- Depth of compacted wheel tracks and longitudinal
- depressions.
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- Heavy vehicles can cause ruts in the pavement over time
- which allow water to pool. The deeper these ruts, the more
- harm to the pavement's ability to drain water.
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- Pavement micro and macrotexture. Because of its resistance
- to local collapse causing ruts and thus allow hydroplaning,
- concrete is often preferable to hotmix asphalt in this context,
- though this depends on the age of the surface and the
- construction techniques employed while paving. The concrete
- requires special attention to ensure that it has sufficient
- texture.
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- Pavement cross slope and grade. Cross slope dictates the
- extent to which the cross-section of a road resembles an
- upturned U. Higher cross slopes allow water to drain more
- easily. Grade is the steepness of the road at a particular
- point, which affects both drainage and the weight of the
- vehicle. Vehicles are less likely to hydroplane while traveling
- uphill, and far more likely to do so at the trough of two
- connected hills where water tends to pool.
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- Width of pavement. Wider roads require a higher cross
- slope to achieve the same degree of drainage.
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- Roadway curvature.
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- Rainfall intensity and duration.
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Factors that affect a vehicle's sensitivity to water depth
include:
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- The driver's speed, acceleration, braking, and steering.
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- Tire tread wear and contact patch shape. The longer and
- thinner the contact patch, the less likely a tire will
- hydroplane. Tires that present the greatest risk are wide,
- lightly loaded, and small in diameter. Deeper tread
- dissipates water more easily.
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- Ratio of tire load to inflation pressure. Underinflated tires
- are more prone to hydroplaning, especially as vehicle weight
- increases.
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- Vehicle type. Combination vehicles like semi-trailers are
- more likely to experience uneven hydroplaning caused by
- uneven weight distribution. An unloaded trailer will
- hydroplane sooner than the cab pulling it. Pickups towing
- RVs present similar problems.
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There is no precise equation to determine the speed at
which a vehicle will hydroplane. Existing efforts have
derived "rules of thumb" from empirical testing in the
1960s and 1970s.
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In general, for cars, hydroplaning can be expected at
speeds above 45mph (72kmh), where water ponds to a
depth of at least 1/10 of an inch (2.5mm) over a roadway
length of 30ft (9m) or more. With much higher tire
pressures in trucks, it is at higher speeds.
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Response
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What the driver experiences when a vehicle hydroplanes
depends on which wheels have lost traction and the
direction of travel. If the vehicle is traveling straight, it
may begin to feel slightly loose. If there was a high level of
road feel in normal conditions, it may suddenly diminish.
Small correctional control inputs have no effect.
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If the drive wheels hydroplane, there may be a sudden
audible rise in engine RPM and indicated speed as they
begin to spin. In a broad highway turn, if the front wheels
lose traction, the car will suddenly begin to drift towards
the outside of the bend. If the rear wheels lose traction,
the back of the car will begin to slew out sideways into a
skid.
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If all four wheels hydroplane at once, the car will
slide in a straight line, again towards the outside of the
bend if in a turn. When any or all of the wheels regain
traction, there may be a sudden jerk in whatever direction
that wheel is pointed.
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Recovery
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To recover while traveling in a straight line, the driver
should not turn the steering wheel of the car or apply the
brakes. Either action could put the car into a skid from
which recovery would be difficult or impossible.
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Instead, with no change in steering input, the driver should
gently ease pressure off the accelerator. Control should then
return. If braking is unavoidable, the driver should lightly
pump the brakes until hydroplaning has stopped.
pump the brakes until hydroplaning has stopped.
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If the rear wheels hydroplane and cause oversteer, the driver
should steer in the direction of the skid until the rear tires
gain traction, and then rapidly steer in the other direction to
straighten the car.
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Prevention by the driver
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The best strategy is to avoid as many contributors to
hydroplaning as is possible. Proper tire pressure, narrow
and unworn tires, and reduced speeds from those judged
suitably moderate in the dry will mitigate the risk of
hydroplaning. Avoidance of standing water is another
effective prevention strategy.
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Electronic stability control systems cannot replace these
defensive driving techniques and proper tire selection.
They rely on the same braking mechanism at the driver's
disposal, which in turn depends on road contact. While
stability control may help recovery from a skid when the
vehicle slows enough to regain traction, it cannot prevent
hydroplaning.
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