Asphalt paving over timber decking is a standard practice for timber bridges. Two publications that are free from the Wood in Transportation Website and FPL are "Guidelines for Design, Installation and Maintenance of a Waterproof Wearing Surface for Timber Bridge Decks" and "Asphalt Paving of...
Asphalt wearing suface with a geotextile fabric. Look at this website for more information http://spfnic.fs.fed.us/wit/WITpubs/PubSearch.cfm
and use a catagory of surfacing.
The asphalt surfacing should give you the best life for your decking but you can use a treated timber wearing suface...
