Hi Cassie
Melbourne Water in Australia have good guidelines on constructed wetlands just look them up on the web. City of Melbourne WSUD guidelines, Virginia DCR Stormwater Spec, City of Portland Stormwater Manual and California Department of Transport Website are useful.
Although what you describe might suit a bio-infiltration trench/basin setup better as roads often carry heavy metals and toxins that can damage fragile eco-systems.
Alternatively you can design in an entire treatment train starting with roadside swales – infiltration trench – bio-retention basin and then a constructed wetland. It depends on what your trying to achieve in terms of local water infiltration or a complete water treatment cycle.
Hope This helps
