When forming a statutory road association about 6 years ago we looked at this scenario with the attorney reviewing our bylaws.  We had one main road in for access that we were focused on for the association.  There existed 4 other roads that came off the maine road. The possible structure the attorney said could be done is to clearly state the purpose of this association and what it was to encompass.  Each of the roads off the main road would have to have a road commissioner who would prepare their budget to submit each year.  The treasurer would have to assign the correct fee to the lot owners based on the submitted and approved budget. The fee would consist of the main road maintenance plus the additional fee for the specific road for the lot owner along with insurance and administrative fees. This could easily be managed with a spreadsheet where numbers could be plugged in for the calculations.

In the end we did not pursue this due to other reasons related to the additional roads restrictions they had in place that we did not want to deal with the details at the time.  I would suggest a review by an attorney to see if this could be done still done today and would make sense in your situation.  While bylaws are not required for a statutory road association in this case they would be useful so everything is defined for a more complex situation.