well, believe it or not, for scenes where there is like a wall and a road, i simply just add smooth surfaces, in blender that is done by pressing space in the left window then add>mesh>plane, stretch it out and add a texture you find at google ir whatever http://imajr.com/350z-render-1164315
like in this one, but the road simply makes me wanna puke
Ambient Occlusion is a visual effects that simulate global illumination shadowing. It has no relation to the physical lights direction / brightness etc. And it does not light up the scene. It is done by calculating the distance of 2 objects, the closer they are, the stronger the shadow will be. The further they are, the softer and lighter the shadow will be.
Ambient Occlusion is mainly used for adding refine details to your render, and believe it or not it is a quicker way to fake Global Illumination.
The way most people use AO is to render it as a seperate pass (a black and white image with just the details of shadows, it renders very quick) then composite the AO layer on your original render in photoshop. Therefore your original render could be done in Default Scanline Renderer without any G.I. process, which will render much faster.
But there is also a downside of using AO. IF you place a light between 2 objects close to each other, they will still generate a harsh shadow between them even though there is a light between them. Because as I said earlier that AO does not react to any physical light at all. Also it does not know the object is solid or transparent, it still can cast strong shadows on transparent object such as glass