A regular Ford Mustang is 140cm high. Depending on the height of the chassis and the wheels that may vary another 10cm. This means each of your characters is a midget. Your curb looks like it's 30cm high and your tree's would catch every truck passing by. I wouldn't really make them bigger, but I would definately…
@SimonT : Yep! Well, it's not Crysis, it's just something that will looks like. UDK is the only engine I know, but I'll definitely check out CryEngine 3. ;)