New York Consolidated Laws, Highway Law - HAY § 131-b. County highways and bridges

1. A board of supervisors may, when satisfied that it is for the interest of the county, lay out, construct, open, alter or discontinue a county highway therein, or cause the same to be done, and construct, repair or abandon a county bridge therein, or cause the same to be done when the board shall deem that the interest of the county will be promoted thereby.  All expenses so incurred shall be a county charge, and may be financed pursuant to the local finance law.

2. Whenever the board of supervisors of a county shall determine to construct a bridge in accordance with the foregoing provisions of this section, such board, on behalf of the county, and the town board of a town or in case of a city the board of aldermen or any similar board exercising the functions of aldermen, on behalf of such town or city, may enter into an agreement with the county, to the effect that such town or city will operate and maintain such bridge, in case the bridge is located wholly in a town or city.  In case the bridge is constructed over a stream forming the boundary line between two towns or two cities or between a town and city, then they may agree with the county to operate and maintain such bridge jointly, in proportion to the assessed valuation of such town or city.  The sum which the town or towns, city or cities are obliged to pay under such an agreement is a charge upon such towns or cities and shall be paid as other town or city charges are paid.