Many years ago I road through a mini-flood downpour. Not the nearly 5" that drowned Toronto, but a fun storm. That was it for a long time. In our old midwestern climate a genuine deluge was really not that common. They were easy to dodge, and with fenders and some gear routine rainfall is no problem.
Things are different now. In the post-400 ppm CO2 world my weather is warmer and wetter; we are told to expect really heavy downpours more often. Like the one I hit last week on my routine commute. Instead of waiting for it to pass I rode on, and so I found out what gear worked and what gear didn't.
- Old baggies: They degrade with age. The ones I had in my front bag were worthless
- My brakes and my vision: Of course both brakes and vision degrade in rain, but in a deluge they really do nothing. This was a bit of a problem because in 3" of a groundwater collection my tires didn't have much traction, and I couldn't see a big pothole under the floodwater. So I went down.
- My GoreTex shoe covers: They didn't work because I didn't put them on. That was dumb -- my shoes didn't dry off for a couple of days. Even though the rain was mostly fun, it would have been nice to have dry shoes for my return trip.
- My panniers: They do well in routine rainfall, but I knew they weren't waterproof. I poured a half-inch of water out of my front bag. My wallet, keys (no electronic fob!) and several maps were soaked; I tossed the maps.
- My waterproof iPhone 5 case: Best $15 I've ever spent. If not for that case my phone would have been ruined. I leave it in my bike bag.
- A rubber lined "conference bag" that I was carrying my work laptop in the rear panniers. This was dumb luck, I had no idea that bag was so water resistant. If I'd been thinking I'd have waited out the heavy rain rather than chance losing the laptop.
- Fenders: No wheel tracks, though in that amount of rain I suppose they'd have washed off.
- My yellow rain jacket and lights; Not sure the lights were visible, but I think the jacket was. In any case I opted for the sidewalk when the road narrowed, figured drivers couldn't see at all. I wasn't that worried about staying dry, but the ancient Nashbar jacked did that too.
- Helmet and helmet cover: I bonked the helmet when I went down, which made me feel better about having it. Helmet cover worked better than I'd expected.
- Synthetic clothes: Wow, that stuff dries well.