I wonder if it makes marketing sense to have a single "car free day" or "bike to work week" or whatever.  Today, we had thunderstorms.  I'm a pretty big nut, but I'm not eager to ride my bicycle 15 km through a thunderstorm to work when I can take my car.  Yesterday was nice, and I rode the bike to work, then after work took the bus downtown to the library, stopped by Starbucks, took the bus back to work, changed clothes and rode the bike back home, in the dark making use of my very expensive and very nice metal halide arc lamp. Very nice. But that wasn't "world car free day."  I can imagine, possibly, Americans driving less often, but you'll know peak oil is serious when Americans bike to work during thunderstorms as a matter of routine.  

It is early fall in the northern hemisphere, but already the days are getting short and the temperatures often cool.  Isn't the USA bike to work week in early spring, when it's rainy and cool?  Even if you pick a better season, you can't predict the weather more than a day in advance.  "Hey eveybody, today's the day!  Today is....well, if it wasn't for the storms it would be a good day to leave the car at home...."  Or do the people who organize these things live in places like Phoenix, Arizona, where in fall the temperatures finally drop into the lower 30s C and it never rains?  

What do European or Chinese cycle commuters do when it rains?  Do they all just cram onto trolleys?
I've see photos of them wearing ponchos while riding in the rain. Did that myself a few times when I didn't have a car.
I commuted for five years by bike, rain or shine. All you need is good rain gear.
You find people who get put off by rain are not very commited to biking in the first place, and as a result haven't brought waterproof gear.

I bike->train->bike to work each morning insted of buying a car are doing a 42 mile round trip. When i have gotten lifts from work mates ive always felt like i have not properly woken up.

I agree with Beige about peak oil being serious when people have to bike in to work be good fun to have more people joining me in the morning :) assuming they still have a job :(

I've never minded getting rained on, what bothers me is that wet brakes can get you killed in a hurry.  So I'll bike home in the rain, but if it is raining when I wake up, I'll walk or take the car.
I've seen a very noticeable increase in people asking me about my bicycle and my commute, but no signs of any more people actually joining me in cycling.  Now that the cold, dark season is about to begin, I'm not expecting much.

Rain on a hot summer day can be nice.  On a cold, dark, fall night, not as much fun, but, yes, certainly doable.  Chance of light rain or even actual drizzle don't deter me, but I'm not motivated enough yet to set out into a downpour with continued rain and storms forecast.  And yet people here think I'm some sort of cycling nut.  (I see studded tires in the bike shop, so there is plenty of room for me to became a real nut someday.)

I do combine the rainy day drives with running the errands that are far easier to do with the car.  Most of the places I want to go to are much closer to work than home, most of them in between work and home, so it's a reasonable trip combination.