There are many proposals to place caps on top of freeway trenches. Today's LA Times features one of the proposals being promoted by the Park 101 group. This story follows a recent USA Today story that looks at many of the freeway cap projects underway. This one in Dallas is actually featured as part of the ARRA funding.
Since freeways are not nice neighbors to have, these are good projects. It is unclear if they can lower the localized air pollution of the roads, but increasing parkland in cities such as Los Angeles or New York is worthwhile.
Streetsblog has more about potential freeway caps in New York.