First problem what you find is how to get correct object scale. So, here I show few basic objects scale, what you must keep in mind:
- Typical human height is about 1.70 - 1.80 meters.
- One house floor up to 3 meter.
- Usually one building floor have about 3 meter. First floor, also usually have 4 m, if you have living house with a lot of flors.
- Street lights pole 10-12 m.
- Small tree 5-10 m. Big tree for 20 to 30 meters.
- One road side have 3-4m wide. So, two sides have 6-8m.
- Road sign have 3m height. And Information face have 0.70 m wide.
- Street lights pole have height from 10 to 12 meters.
- Light Cars - approximately 3 meter long, 1.5 m height and 2 m wide.
- Young - 5, 7 meters
- Trees 20 years old - 10 - 15 meters
- Old big tree, forest - 20 - 30 meters.
Modeling process :Recommend using array modifier together with curve modifier.
With bezier curve, need to draw for basic road, fence or any repeating objects path.
With "Mifth" addon and any other yuo can easy plant yours object around you project.
"Drop to Ground" addon is good for single object place right on background...
TIP:So, if you know some basic scale, you can use it as orientation to decide about object around it, especially if you don't have possible measures from place and have just photo or video references.
Next important thing, what you must keep in mind, is try always seeking life around you. Yes, right, and put all in your memory. Looking what is vegetation in forest. What you can find around road, what is grass here, what flowers blossom at this time. If you don't have time for walking or not to much driving, you can watch on board video. And you always be sure, that grass, which usually living in forest (like fern grass), you don't put in open areal and so on...
Again, repeating, especially if you placing vegetation, you can use just few objects, but if, also you will change rotation and size, on each. Finally it will looks natural with big variations. The same rules is good for other objects as line of tyres, flags and other objects which can repeat, in your situation. In real life is very hard to find objects which have the same location and orientation. So, don't place it mechanical, be an artist, artists of "faking life".
Also, what I talked about your personal photos base. Usually I use my personal taken pictures of trees, grass, cars or spectators. If I don't have enough, I try find it on Internet or just draw it (as textures)
But you can use native object or textures from game, too. I usually use just vegetation textures if I want save time. But must to keep in mind, about permission if want use textures made from independent track creators (read in track credits)...