UE4 goes by centimeters (sometimes its labelled as Unreal Units, but 1 UU = 1 cm). If your road is 5 meters, and you want a completely accurate recreation, then make the road 500 centimeters.
The thing is, you shouldn’t expect this is work out perfectly. You’ll need to adjust the scale based on you’re system’s capabilities. Having an exact city replica is great, but it might be too taxing on your system.
~ Jason