Heavily travelled roads are paid for by the municipality/county (using your tax payer dollars of course). As far as streets, sidewalks, and curb improvements, often the city will levy special assessments against a property or increase the property taxes to cover the cost. The justification is that the improvement increases the property value of your home/building, hence the tax increase.

Bonds are issued to finance the public infrastructure improvements and the local assessments to the propery owners go to pay back the debt service on the bond. The local assessments are necessary as general tax dollars do not pay for streets and sidewalks. The local assessment is determined based on the value of the improvement to the property. That value is determined by the local municipality.... naturally.