The itchy-bump leg rash that is very painful is almost always something called PMLE (Polymorphic Light Eruption). It really isn't *caused* by reflective pavement, but that kind of pavement tends to make the reaction worse because it reflects UV radiation. PMLE is believed to be caused by an immune reaction to a compound in the skin which is altered by exposure to ultraviolet radiation. The more fair-skinned you are, the more likely it is to get you, and it is most often grown women who are affected. Here is a nice layman's explanation from the New Zealand Dermatological Society:
http://dermnetnz.org/reactions/pmle.html I starting getting it about 10 years ago, but it doesn't happen to me anymore because I take steps to prevent it. I can tell you that prevention is *crucial*; once it erupts the only thing that will really get rid of it is time.
With PMLE, you need to spend time in the sun before your trip as much as possible; even sitting near a window wearing shorts will help, if it is cold outside. (Some people even use tanning booths to do this "hardening" process, but I think that cure may be worse than the disease.) Also, start a course of oral antihistamines several days before the trip, and keep taking them as long as you are touring (I use Claritin.) Be sure to also make generous use of a broad-spectrum sunblock whenever you are in the sun, and reapply about once an hour. (It doesn't have to be hot outside bring it on, PMLE can erupt if the sunlight is just "brighter" than what your skin is used to.) BTW, antibiotic cream and things like calamine make a PMLE eruption worse; don't try them. Stick to OTC hydrocortisone or PrameGel if you have to treat an eruption.
There is also another similar condition that a lot of people call the "sock-line" rash; that is a form of edema from being on your feet more than you are used to. This one usually is not really itchy or painful, just unsightly, without raised bumps, and you can recognize it by the clearly defined line where your socks stop. Soaking your feet in cool water and keeping them elevated as much as possible will help with that.
Last but certainly not least is prickly heat, which is not the same as the other two because you get it in areas where your skin is covered or folded, rather than exposed. The trick to preventing prickly heat is to keep your skin dry, so wear light-colored clothing that dries quickly and wicks moisture away from the skin, and also try to reduce or eliminate friction of clothing against the body, or skin against skin. BodyGlide is great for helping to prevent prickly heat.