The State of Maine, Bureau of Bridge Maintenance has developed a Best Practices agreement with their Department of Environmental Protection on the cleaning and washing of bridges. Some states have similar agreements in place while others have not developed this type of agreement.
As the washing and cleaning of bridges is considered, a more global environmental perspective should also be considered, i.e., the environmental cost of replacing a structure. The environmental impact / cost to manufacture steel and concrete today is considerable and the premature replacement of a bridge only increases the overall impact.
The attached link will take you to the Maine DOT bridge website, where you will find Bridge Maintenance’s Best Practices for bridge cleaning and washing and also a letter from the Maine Department of Environmental Protection allowing the Maine DOT to perform this operation.