I can only speak for what I do: Transportation Dept. Normally, Landscape Architect will prepare a construction document. After completion, the project will be advertised and qualified contractor can submmit their bids. The lowest bid will get the project. Construction document including plans, and specifications will have to be quite precised to avoid change orders during construction phase. You can read a bit more via this weblink:
http://www.dot.ca.gov/hq/esc/oe/weekly_ads/index.php