If you race in one country the signs may be in miles. If you race in another country the signs may be in kilometers.
Since the only sign that you need to obey is the pit speed limit then there's no big problem here really for most people, I would imagine. If you wish to change the signs yourself you could try editing the DDS files.
Currently the speed signs occupy image files with other textures and signs used in game. Changing the speed signs would mean making duplicates of all the existing DDS files that contain speed signs (and distance signs leading up to a corner) or else making new texture files from the beginning. Furthermore some tracks have different signage than others.
So this is not a simple thing as far as labour intensity goes.
If absolute reality is your dish then perhaps the speed signs in LFS should just refelct the country they are (virtually) located in? This would not give you your magic changing signs of course.
Most of the time I don't even notice any speed signs, I'm too busy racing. The braking markers (distance signs) leading to a corner don't need to be any particular measurement really, numbers are numbers.
So until or unless you ever get this feature in LFS you may like to try to edit the signs yourself (Photoshop or Paint Shop Pro handle DDS files after installing a small plugin).
As for the speed limit in the pit lane, 50mph (49.71) or 80kph is the limit.