They could cover the Allen like this:
http://www.theweathernetwork.com/news/storm_watch_stories3&stormfile=a_greener_highway_for_german_041211?ref=ccbox_homepage_topstories
A greener highway for Germany
Hilary Hagerman, staff writer
December 4, 2011 — Germany's Autobahn 7 is a notoriously busy and loud highway. But it's about to get a bit greener.
A rendering of what the park will look like
Germany’s longest motorway is about to undergo a green transformation.
Autobahn 7 (A7) is the country’s longest motorway, measuring a lengthy 963 km. Soon, a stretch of the highway will become a giant public park.
Starting in 2012, construction crews will begin to build a 10-foot tall canopy over about 5 kms of highway – starting in Hamburg’s Scnelsen district, running down to Stellingen and ending in Bahrenfeld.
This strip of the A7 currently divides Hamburg’s Westside from the rest of the city, and sees about 150,000 cars per day.
Construction will start with the Stellingen district, where the kilometre-long park will boast a wooded parkland and community gardens. The half-kilometre stretch in Schnelsen will see a tree-lined promenade to offset the noise caused by six lanes of traffic underneath. Bahrenfeld will get the longest stretch of canopy – and will even see the 1,700 new residential apartments above the A7.
It will take about four years to finish construction, and will cost about a billion dollars. Germany’s federal government is funding the bulk of the project.
The greenspace will be built over the existing highway
It won't be an easy task. In some places, the canopy cover will need to be over 100 feet wide. And the idea hasn’t come without its share of controversy. Many say that it will help reconnect districts that have been divided since the highway was built almost 30 years ago. But others say that construction of the parks is bound to disrupt the flow of traffic on a major highway that connects Denmark and Austria.
Also, gardening plots in Bahrenfeld’s southeast side will be forced to relocate because of the development.
This stretch of highway won’t be the first to be covered in Germany. Other canopies have been built in Munich and Dusseldorf, but this stretch would be the largest of its kind.