Well, I think your faith in technology to solve our problems is perhaps a bit inflated. You see, it's one thing to paint a roof or roadway white. It's quite another to come up with a white paint for a roof or roadway that won't be slippery when it's wet and that will stand up to decades of direct intense sunlight beating down on it like tar and gravel roofs and asphalt or concrete roadways do.

The reason why they put pea gravel on top of flat roofs is to protect the underlying asphalt and roofing felt membrane from the UV light of the Sun. It's exactly the same reason why shingles are covered with granules. The aggregate's job is to block the intense UV light from the Sun from causing the asphalt (or asphalt shingle) to deteriorate.

I suppose using white-ER pea gravel on roofs would accomplish the same thing as painting the roof. but getting a paint to last as long as asphalt or concrete when exposed to the same conditions as an asphalt or concrete roadway is going to be difficult. For example, could you still run a snow plow or street sweeper over that white paint without wrecking it? How about putting sand down at intersections during the winter? Won't that sand wreck the paint come spring? And, of course, brand new tires are safe because of their deep treads for accomodating water on the road, but those deep treads also collect stones that will chew up white paint on the road way.

And, on top of all of that 2/3 of the Earth's surface is covered by water which you can't do much about, and the vast majority of the land IS NOT covered with flat roofs or roadways. Painting a shingle roof is asking for trouble because then it'd be slippery when it was wet, and you'd have people slipping and falling off sloped roofs all over the place, and it's those sloped roofs that account for most of the roof area in any town or city.

PS: I always get a kick out of looking at pictures taken outdoors in the southern USA cuz the shadows are so short that you have to look twice to even see there's a shadow! I notice that during sporting events broadcast from the southern US too. I also like to see people wearing light fall jackets outdoors in December. No touques, no gloves, no ear muffs, no electrically heated socks, just a light fall jacket. That's why most Canadians look forward to watching the Superbowl if it's held in Florida, Texas or Southern California. The shadows, the clothes the fans are wearing, and of course, the commercials.