I’m tentative about asking stuff in ‘General Questions’ (because in this case, I’m not sure if there really is ONE factual answer)…
Suppose I’m driving up the freeway, blowin’ and goin’ at my own speed with little traffic… am I getting better “traction” an hour after it started pouring rain, or am I better off gunnin’ it in a light, steady sprinkle, to try and get home before the real shit starts?
In my head, the light, steady sprinkle doesn’t freak me out as much as the shit-pouring rainstorm, but I could’ve sworn that I saw or read somewhere that the most dangerous time is “right after it starts sprinkling, with all the oil, treadwear, and slickness on the roads starting to get wet”
Is there one factual answer to this? For the sake of “argument”, I’m leaving out the idea of ice/snow/whatever - let’s pretend it’s a 75-degree day.
“right after it starts sprinkling, with all the oil, treadwear, and slickness on the roads starting to get wet”*
That is the worst time. Many years ago, as a student in London, I worked for a garage at weekends. My job was to work the radio and send recovery trucks to breakdowns and accidents - we normally had three on call.
If it started to rain, the phone would start ringing and we would be dragging bent cars in as fast as we could. The storage area would quickly fill up and when I left for home, they would be lined up along the road outside.
Anyone who follows F1 racing will also have seen how the track becomes a skating rink if they get a shower during a race.
The way I was taught “the first rain or a light sprinkle lifts the oil deposited from cars and makes for a slicker/slippery surface. Be especially careful at lights and stops as they tend to be “gathering spots” for all kinds of leaking fluids. Also be aware of painted lines.”
Especially on a motorcycle, I still follow that advice.
That’s what I’ve always heard, when if first starts raining it brings all the oil up to the surface, after a while that’ll wash away.
Of course, if the choice is “beginning of a light sprinkle” and “middle of the pouring rain”, I’d think a light sprinkle would be safer. It’s probably not enough to mess with the road and it won’t affect visibility. But to be fair, I think you have to make it even. If this were to be tested, you’d have to have four options. Beginning and middle of a light sprinkle and beginning and middle of heavy rain. I think it would be the beginning of the heavy rain that would cause the most problems…based on the premise.
To be honest though, I think you have to hit a pretty specific set of circumstances of ‘rain just starting’ and ‘hitting a slick spot’ and ‘actually having something to hit’ to actually end up having an accident that I’d rather attempt to beat the rain if I knew a downpour was coming. It’s not like driving on ice or heavy snow where you’re pretty much guaranteed to slide around at some point during your commute.
If it starts raining down buckets, to the point that the water literally cannot drain off the road fast enough, I’d say the danger from hydroplaning far exceeds that from a light rain mixing with oily roads (that, and the huge decrease in visibility). Ask anyone what the grooves in car tires are for and they’ll all say, “Traction”, which is incorrect. Completely smooth racing slicks give the best traction on pavement. But that pavement has to be well maintained, flat, and totally dry. If it starts to rain and they want to continue racing they must change to rain tires (tires with at least some grooves in them).
I have heard the ‘early rain is the slickest’ supposed fact throughout my life. I would be willing to accept that it is true for that one factor in a limited set of conditions but I don’t think it is the most important factor and I haven’t seen any good data for it that would universally apply.
Every place that I have ever lived has above average amounts of precipitation and there isn’t a noticeable buildup of oily, slick conditions that rise to the surface during the early parts of a rainstorm or drizzle. That applies to everywhere from Louisiana to New England that I have personal experience with. It could be true in dessert conditions but I have little experience with those.
You also have to define ‘heavy rain’ carefully. I have been in rainstorms so heavy in Louisiana in particular that they reduced visibility to zero and produced flash flooding on the roads. That is hundreds of times more dangerous than general slick conditions. Even normal thunderstorms in the South and Midwest can be so quick and intense that driving at anything past a crawl is impossible for a time at least unless you have a death wish.
I drove for twenty years with a courier service in the SF Bay Area. Typically, there is no rain from about late March to Oct/Nov. and heavy traffic in general. First decent rain every year resulted in (literal) carnage on the roads.
Radio traffic reports couldn’t keep up with the number of accidents.
I’ve also always heard about the whole “oily roads after it just starts raining” but I’ve never really seen it. I suspect it was something that was a lot more common back in the days of draft tube crankcase vents and when cars just leaked more oil and unburnt hydrocarbons onto the road in general. I can see it still being an issue in places like California where you have the combination of ridiculously high traffic volume and months without rain, but I’m not sure the impression given by driver’s ed programs that it’s a common situation is all that accurate today.
Don’t discount psychological factors in that phenomenon. It happens in a lot of places when the conditions change from one season to another. In New England, people commonly remark about the same thing during the first snowfall of the season no matter how minor. There are supposedly an unusual number of wrecks due to drivers somehow forgetting how to drive in the snow from year to year even if they have done it their entire lives. They get better after that until the next year rolls around.
Like I said, I can believe oil rising to the surface early in the rain in an area with infrequent precipitation could be a minor factor but I have never witnessed it personally despite driving over 600,000 miles in 30 states over the years. Wet pavement is always going to have some slick qualities but it is fairly easy to deal with if you are a competent driver - much easier than ice and snow. Like Greasy Jack said, there isn’t nearly as much stray oil spread across the roads as there was 30 or 40 years ago and the road surfaces themselves are better overall. Cars in general are much better too. My small SUV with traction control, 4wd and all-season tires can easily drive up a steep ice and snow covered hill without even slipping a wheel. I have had to do it multiple times a day this Winter. Any type of regular wet road surface is sandpaper by comparison.
How long has it been since the last good rain? As other’s mentioned, things like oil and dust can build up on the road if it’s been dry for a few days or longer. That stuff can get slick when wet.
How good are your tires? Tires near the minimum tread depth are likely to hydroplane - again as other’s have mentioned - which means that the tires are no longer in contact with the road. There’s a sheet of water between the tires and road which is no good for traction. Plus all tires aren’t made the same. Some cheap tires, such as those being imported from China, are horrible in the rain. On the other end of the spectrum are some high performance tires that are made for high speed driving on dry roads.
Either way it’s better to slow down compared to a nice, dry day. And have good windshield wiper blades!