Cars, buses, and rogue pedestrians are all conspiring against cyclists in congested cities, forever running them down, scaring them silly or simply getting in the way. It's something designer Norman Foster – an avid rider – hopes to alleviate with a dedicated biking highway built above London's rail lines.

The purely hypothetical but nevertheless amazing SkyCycle would stretch 137 miles in and around the city, accommodating as many as 12,000 riders per hour on a cycling superhighway 50 feet wide. The dream calls for 200 on- and off-ramps which, according to Foster + Partners' estimates, means nearly 6 million people will live or work within 10 minutes of an entrance. Without all those cars to weave around and lights to stop for, travel times to and from work would be reduced by up to 29 minutes.

"SkyCycle is a lateral approach to finding space in a congested city," says Foster. "By using the corridors above the suburban railways, we could create a world-class network of safe, car-free cycle routes that are ideally located for commuters."

Putting aside the cost and difficulty of building a 137-mile highway above a working railroad – a point the folks at Foster + Partners sidestepped entirely – we will note that such a highway would be bike-friendly. The railway lines were built for steam trains, so the grades are minimal; the lines follow the natural contours of the land; and – most importantly – the space above them is underutilized, particularly in industrial areas. And according to the proposal, the elevated bike paths are also cheaper to build compared to traditional roads and tunnels. Not that there's any space for new asphalt to begin with.

"SkyCycle is an urban cycling solution for London," says Oli Clark, who brought the idea to Foster over two years ago. "[It's] a cycling utopia, with no buses, no cars and no stress."

Dedicated cycling roads aren't new, of course, and the bike-loving Danes went all-out with an 11-mile road specifically for riding and commuting. The road, built in 2012, even has air pumps along the way.

A raised platform for riders has been proposed in other cities. Way back in 1899, entrepreneur Horace Dobbins of Pasadena, California, joined former state governor Henry Harrison Markham to create the California Cycleway. The first 1.3-mile wood cycling structure was opened in 1900, connecting two hotels in Pasadena. The goal was to expand all the way into downtown Los Angeles, but the Cycleway was dismantled a few years later because it was so unprofitable. The land eventually got paved over to create the Pasadena freeway. Blame Ford's Model T.

The SkyCycle design team is trying to raise money for a feasibility study that includes a four-mile route between Stratford and the Liverpool Street Station. That section alone is estimated to cost £220 million. If it works out, Foster believes SkyCycle could be a reality within two decades.