Except as otherwise provided by law all town bridges not constructed with town bridge aid funds and which the town has the duty to maintain shall have a carrying capacity of at least 6 tons. The selectmen shall cause notice of the load limit of such town bridge to be posted on or near the entrances of such bridge. Such town bridges may be posted for a load limit of less than 6 tons but not less than 3 tons when application has been made for town bridge aid pursuant to the provisions of this chapter and action on such application is pending.