How much should you reduce your speed in rain?

Answer 1

That depends on many variables. Vehicle and tire type and condition road and rain condition etc.(SUV's and 4 x 4's DO NOT handle better at highway speeds). Most important to remember is your ability and how comfortable of driver you are. Drive at a speed that matches your comfort level. Never over drive your ability. Never test your abilities on busy public roads. Learn your vehicles safe stopping distance in a safe and secluded place and double or triple that in the rain. Remember the 2 second rule (2 counts behind the vehicle directly in front ) and double or triple that in the rain. professional training is cheap compared to an insurance claim and one or two lessons can make a big difference.

ANOTHER VIEW of safe speed in the rain.

Many Factors may be considered.

Avoidance. Pay attention to weather forecasts. Heavy rainfall storms usually soon pass so consider postponing journey or getting off the road to wait at safe place. Your perfect driving may not prevent another car smashing into you.

Visibility can be severely reduced. more so in dark. So help it with clean windscreen (inside also) efficient wipers, rain repellent agents, clean eyeglasses where anti reflection coating help. Use lights as recommended.

UK Driving Standards Agency publications give advice on increased stopping distances and increased spacing from any car you are following.

Unusual hazards which heavy rain may bring. Sand, mud, even gravel or rocks may be washed on to road. Parts of road may be washed away or blocked by Landslides. Leaf fall from Autumn or storm winds. Mud from farm vehicles. Wild deer, Farm animals and their droppings can be unexpected hazards on smaller country roads requiring slower speeds in wet. Light Rain after long dry period can make road surface unusually slippery.

Awareness of aquaplane (or hydroplane phenomena. A speed that matches drivers comfort level will prove, I suspect, often dangerously fast, particularly as with modern car comfort and handling we have become used to ever higher speeds. November 1009 brought unusually heavy rain in UK with many reports of aquaplaning conditions and incidents including deaths.

Areas of standing water occur when heavy rain cannot drain away fast enough perhaps retarded by wind. Depth of water then may be greater than guessed and deeper than the best tread patterns can possibly handle - particularly when road surface has depressions from heavy road wear. When rain drops appear to bounce (splash up) there must be significant standing water.

Typical motorway speeds are well above that at which the tyres in such conditions will lift clear of the road surface and have no contact so that neither steering nor braking is possible. ABS and ESC will be no help until car is slowed by wind (and water) drag. There is advice to slow to a safe speed but just how slow is necessary to keep tyres in contact. I suspect that attentive and experienced (ie. on wet roads), and lucky, drivers may subconsciously learn from minor reduced control incidents in a range of conditions. Less experienced drivers will be at an unfair disadvantage if limits of tyre performance have not been taught or discovered safely. For just how slow I draw attention to reports by NASA and Autocar detailed below.

Towed trailers can be susceptible to the phenomenon. They often have tyres at lower pressures and may also have a more vulnerable contact shape particularly if lightly loaded and with smaller radius.

NASA published in Nov 1963, TN D 2056 "Phenomena of Pneumatic Tyre Hydroplaning" by W.B. Horne and R.C. Dreher.

Both aircraft and road vehicles were objects of the experiments.

They found that when water depth exceeded tread depth full hydroplaning occurred close to speeds predicted by a formula (see below). Forty years later car tread contact shapes have changed (wider -more vulnerable?) and a factor of 9 in the formula instead of 10 gives better agreement with Autocar tests of car tyre performance (see below) thus: Critical speed mph = 9 x square root of tyre pressure (psi).

This formula gives 54 mph when tyres all as high as 36 psi and approx 50 mph when tyre pressure 30 psi. But caution here. This does not mean you are always safe below these speeds but that you WILL lose all control above them when water exceeds tread depth.

Autocar of Nov 1999 published results of Tyre Tests on a flooded race track at Mireval, France. Various car tyres showed critical speeds varying from approx 49.7 mph for the best tyre to as slow as 44.7 mph at onset of hydroplane. Water depth was 7mm.

I have found in talking to many drivers after the rains of November 2009 (UK) that few had clear idea of critical speed for their cars. Several mentioned brief incidents which had taken them by surprise. Ideally skid training could include experience of the hydroplane phenomenon.