There is weather that no-one should be riding in - thunderstorms, tornados, tropical storms, icy conditions and high winds. The reluctance of riders to set out in bad weather is well known. We often find ourselves navigating around storms and changing our schedules to avoid rain. In Europe, home of the Maids study, bike and scooter commuters commonly have fall-back plans for bad weather. Public transport ridership increases on rain days, and many riders have fallback carpools. In the USA, where riding is more of a pleasure activity, local rides are often cancelled for weather, and some riding groups have rules to that effect.
We think that the reduced number of riders in the rain masks the effect of bad weather on accident statistics. Unless a study makes a major effort to estimate reductions in ridership during the rain, the studies won't notice an increase in crashes per rider-mile in the rain. In fact, there might be a reduction due to fewer riders. We think that the Southern California location of the Hurt study had a lot to do with this, and the omission was repeated in Maids, but was corrected in Thailand, where the monsoon months bring the subject of rain to the forefront.
The Thailand study had an enhanced population control method, which accounted for rain, and found:
Rain was an infrequent cause factor because most riders did not ride in the rain... However, when it was present, adverse weather often contributed to accident causation. In the 18 cases in which the weather was inclement (i.e., raining) it contributed to accident causation in 12 of those cases, usually by limiting the rider’s ability to see.
-- Motorcycle Accident Causation And Identification Of Countermeasures In Thailand, V.I. Kasantikul, 2001.
We also think that we quickly train ourselves in what's needed for rainy riding. If we ride as usual, we'll soon go down. We might survive one or two put-downs in the rain, as the lowered friction and bulky rain gear might allow us to skid to a halt without road rash. We don't recommend experimenting with this. It won't take long for nature to teach us a hard lesson in rain riding.
Wet sufaces mean reduced friction between tires and road. You can't lean as far into a turn, so you have to take them slower. Braking and acceleration has to be more gentle. We need to practice even more defensive riding than usual, because we need more time and distance to speed up and slow down.
Our ability to see is compromised when visors, goggles and windshields fog up and become coated with rain. Manufacturers often say not to use an anti-fog product on polycarbonate and other transparent plastic equipment. In fact, you should not use a product designed for glass on these plastics. The good news is that there are a variety of sprays and wipes on the market which do not harm plastics. I use stuff my optican gives out free, designed for plastic and polcarbonate eyeglasses. These products combat misting and cause raindrops to coalesce, where their greater weight makes them fall off faster. Other folks recommend the diver's remedy, to wipe spit on the plastic. This works, but I never seem to have enough spit for my windshield.
The owners forum for your bike or equipment may have more on what works on your gear, and some equipment manufacturers recommend or supply specific products for their gear.
The Thailand study identified issues with rider visibility, rather than friction issues, as the major cause of rain accidents, but these factors are related. Reduced friction requires more reaction time, and reduced visibility reduces reaction time.
You and your bike are even harder to see in the rain. If you have modified your bike lighting for conspicuity and are wearing bright gear, that's a start, but the actve conspicuity measures in this section become more important too, and you might consider using hazard flashers in extreme conditions. If you can't be seen, maybe it's time to pull over and wait it out.
Getting wet is a comfort factor, and when it's cold also a safety factor, due to exposure. Our injury mitigation section has information on riding gear. If their gear is not rainproof, most bikers carry rain gear. This is an individual matter as bikes vary in offering rain protection via windshields and fairings. Consider getting highy conspicuous gear as you need the extra brightness in the rain.
High winds and gusting cross-winds deserve mention. Unless we are in extreme winds, where we should not be riding, wind generally is not a threat. Winds tend to throw us off course and we might wander in the lane, but most riders soon learn to counter wind gusts and crosswinds. Many new riders think wind is very scary. There are risks involved, especially in highway driving around trucks, and when riding in a group. When these factors are combined, the problem is worse.
Wind is different for everyone. Lighter bikes and ones with fairings and windshields are affected more by wind, and the effects increase with speed. Novice riders generally have fewer of the skills needed to ride a straight line in wind.
Trucks are large windblocks. When passing a truck, you can get sudden blasts of wind through the gap between tractor and trailer,and at both ends of the truck, and these effects can be worsened by slipstream from the truck itself. This gets worse when topping hills and on bridges, and prarieland seems to always have high winds.
We recommend assuming that your bike will wander in high winds and when passing trucks. The best bet is to use the center of your lane. In the case of group riding, this is even more of a problem, as wandering into another bike is an additional danger. If the ride leader does not put the group in single file, any rider is allowed by the protocol to issue the 'single file' hand signal whenever he or she is concerned by wind and/or truck passing issues. It is part of 'riding your own ride'. Your ride leader, if he is paying attention, will probably do something like passing fewer trucks or issuing the 'single file' command himself when he sees riders asking for 'single file'.
If the wind is not bad enough to put your bike down, management of the bike space window to allow for bike wander is essential. In time, riders learn to automatically correct for wind wander, and the wind gets less scary.
A combination of wind and rain makes everything worse, and it is often necessary to slow down as wind corrections consume a percentage of the already reduced tire traction.
If you are on the highway and unable to maintain highway speeds because of wind and/or rain, consider getting off the road until conditions improve. Low visibility is a frequent cause of freeway crashes, and bikes, because of the conspicuity issues we taked about, are even more vulnerable to rear-end collisions in foul weather. We consider riding slower than the average slow-lane speed on restricted-access highways to be extremely dangerous for this reason.