Using a bezier circle as Bevel Object creates the same result.
And if I understand your second suggestion correctly, this would case an uneven spacing to the edge of the lane, because of the round end and corners in some lanes. But moving inwards is luckily the smaller problem, as my current method seems to work in most cases.
I’m currently trying to extend the addon linked by @Safetyman to include an option for automatically scaling the path’s vertices according to the calculation in this post from the previously mentioned thread.