concrete and steel bridges are great - if you don't mind the cost, CO2 from concrete production, and if you have an economy that can provide the materials and expertise to build it
this is the situation in the US (except for the occasional collapse that brings the assumption of expertise into question)
southeast Asian countries now have a a more appropriate option using local sustainable materials, minimal labor, and presumably at a low cost - bamboo bridges. A prof at both U SoCal and Hunan U has designed a 30' bridge rated for 8 ton vehicles using a bamboo structure built by 8 local laborers in less than 1 week. No word on expected lifespan or maintenance requirements, but bamboo should do at least as well as a wood frame bridge, since bamboo naturally resists rotting.
Bamboo is an especially good material because it is a thoroughly renewable resource, rapidly replenishing itself every year. Unlike the trees used to supply the wood for our stick-built houses which take at least decades to regrow, bamboo is a grass and can grow up to 3 feet per day.
This reminds me of a wind turbine that I want to build with a stressed bamboo frame and colorful blades made from a light fabric. It would be made of all sustainable materials, and compliment the architecture it was attached to. I have no doubt that it would be a commercial success even if it never produced a single watt. It would be desired as a dynamic sculpture signaling your dedication to beauty and the environment. The only problem is that most of the customers would be in SoCal and I have no interest in going there.
I'll probably build one at some point. A model, anyway. Here is a paper from an Indian guy on the subject.