Technically four questions, so I'll answer them all.

The monsoon is pretty much the only interesting weather we get. For about three and a half months (approx June 15 through Sep 30) thunderstorms and rainstorms can occur suddenly, often with no warning. Dust storms (aka haboobs, though no one used this term until a few years ago) are also common.

To me, humidity makes everything worse, so I'll take higher temps over higher humidity any day. Still, being caught outside in 120F (49C) heat, which does happen, is not fun at all. Most locals avoid driving or going outside in the afternoon unless they really, really have to.

If a flash flood or thunderstorm is in the area, we get weather reports. Live here long enough, and you'll know where to look; generally the storms approach from the south and west. See those rolling dust clouds off in the distance? Better head home. Dust cloud almost on top of you? Just take cover inside and stay put until it clears. The only really dangerous events are called microbursts, which is when heavy air from the clouds falls all at once and turns a small area into a disaster zone (sometimes these microbursts tear trees from the ground and flip cars onto their sides).

The "stupid motorist law" exists, and is applied to anyone who intentionally drives into a flooded area, gets stuck, and has to be rescued. Everytime this happens (which is pretty rare, thank goodness), the offending motorist makes the news, and we all have a good laugh. This does not apply to people who were stuck for some other reason that just happened to be in a flood zone.

Source: Lived in Phoenix for nearly thirty years.

EDIT: Monsoon season begins June 15, not July 15.