Worse is the 2nd step, after you have the bridge built and this first test, ie. Enabling survived after the construction phase and still has to decide is how resilient the bridge really is and let cars drive over. In the beginning you can pick's and small trucks between normal car. The goal is of course always to get as much load as possible on the bridge that is. the truck's. But I must confess to my shame that I have one or the other had to rely on the time cars.
The level number is in my opinion sufficient and was recently even extended, but as I'm still far from the end remote (I'm a bit stuck with a big bridge ...) I can not say how late game is.
Having just hang on a level I would say that the difficulty quite is fine! It is by no means so that it is every time the same solution and also not so simple that you always make the first attempt.