Alright, this is actually a tough one as it seems to be fairly universally agreed (e.g. insurance companies, safety commissions, state governments, etc.) that cruise control should not be used in the rain or in icy conditions. The ICBC summarizes what most of the arguments against using it are:
Turn off the cruise control: Wet roads can cause wheels to spin and
lose control. The only way to stop wheels from spinning and maintain
control is to immediately reduce power. An activated cruise control
system applies continuous power, keeping the wheels spinning. By the
time you disengage the cruise control it may be too late – you may
have already lost control.
Interestingly enough though, the RAA acutely says this is a myth for effectively the same reasons:
RAA Technical Manager Mark Borlace, said the potential for this
condition to exist in cruise control systems had been investigated,
but the claims were unfounded.
“Cruise control is a device used to keep the speed of the vehicle
constant, by measuring the speed of the wheels and feeding this
information into the cruise control system that regulates the engine’s
output,” Mr Borlace said.
“Should the car’s tyres break traction with the road, such as in an
aquaplane situation, the increase in wheel speed would be sensed and
the cruise control system would then reduce the amount of throttle and
maintain the set speed.”
CarPoint.com.au has an extremely good write up on this issue and directly contradicts one of the key-points in the email,
This is the key that makes a nonsense of the email. Modern cars take
their speedo reading from the driveshaft or transmission. This means
the cruise control bases its responses on the speed of the driven
wheels, not the car itself. This is an important distinction and fail
If the driven wheels skid because they lose grip, the spinning wheels
will cause the speedo to show a higher reading which will force the
cruise control to release the throttle faster than most drivers.
Regardless of whether the car itself slows down or speeds up, the
cruise control will always reduce the throttle no matter what until
the driven wheels slow down back to the pre-set speed.
If the wheels continue to slip under this scenario, this will always
leave the car travelling more slowly relative to the road, not faster
as described in the email.
Most of the internet hoax repositories include this story (Hoax Slayer, Snopes, Truth or Fiction) and have some additional sources that are much harder to verify but they also don't say much beyond what the quoted material does.
Thus, it seems though that while the email is correct in that the universal advice is to not use cruise control in wet or icy conditions, but the cruise control itself will not cause the car to speed up as claimed in the email. However, cruise control may cause the driver to practice unsafe habits and since the cruise control cannot register the loss of traction on the road, if you lose control of your steering it may cause you to accelerate in an undesirable direction once you gain traction again. As noted by the CarPoint.com.au article,
What has cruise control got to do with aquaplaning?
Not much, but it’s
how most drivers react while the cruise control is on that will almost
certainly decide whether a nasty aquaplaning situation turns into
anything more than a scare.
Here are some examples you should consider...
After drivers engage the cruise control and no longer have contact with the accelerator pedal, they can no longer feel if the wheels are
losing traction until it is too late.
When something does go wrong while the cruise control is engaged, most drivers stab the brake pedal to disengage the cruise control. If
the car is starting to aquaplane, hitting the brakes could be enough
to lock up the wheels completely. Even with the wheels locked-up under
brakes and the cruise control disengaged, the car would continue on at
speed. ABS-equipped cars could save most drivers in this situation.
As some drivers move their feet well away from the accelerator pedal when the cruise control is engaged, experts suggest that the
above scenario is just as likely to be caused by the driver stabbing
the accelerator instead of the brake as they tried to disengage the
A car with cruise control engaged does not know to slow down before a corner nor does it know when to reapply the power as you
leave the corner. A cruise control forces the car to rely totally on
its front wheels to steer through a corner when good drivers would
normally use a strategic sequence of brakes, accelerator and steering.
If the front wheels start aquaplaning in the middle of a corner, the
cruise control cannot register the loss of grip unless the car is
front-drive. The cruise control would then keep pushing the car
straight ahead off the road or across oncoming traffic. Applying the
brakes to stop this could make an already dangerous situation worse.
Cruise control can make you inattentive. By the time you wake up to a problem situation and work out what to do, it can be too late.
So to summarize, the email is correct in that using cruise control in wet or icy conditions is not recommended; however, it will not cause to you speed up as described in the email.