Complex sentence: Even if the road (and the bridges) are meanwhile mostly upgraded to concrete, there is still a piece of about 12.5 km in the northern part that is not fortified especially during and after rain many cars are trapped in the mud.
Split into simpler sentences: Even if the road (and the bridges) are meanwhile mostly upgraded to concrete, there is still a piece of about 12.5 km in the northern part that is not fortified. Especially during and after rain many cars are trapped in the mud.