Why is it so important for bike lanes to have physical separation from traffic? Because without it, you never know when a two-ton metal box is going to fly across your path.

The photo above was snapped on 59th Street between First Avenue and Second Avenue and posted to Twitter by Dirk Peters last night. If those Jersey barriers weren't there to protect the westbound bike lane, this might not have turned into a story about bike lane design -- it could have been about someone on a bike getting seriously injured.

Meanwhile, on West Street in Greenpoint, where the Department of Design and Construction has installed seven blocks of the Brooklyn Waterfront Greenway, you can see the other downside to going without protection: Motorists turn the bike lane into a parking lot.

The bike path is in serious need of physical barriers, judging by these photos that Doug Gordon posted on Twitter earlier this week. Between Noble Street and Milton Street, the entire path was filled with parked cars.

There was some of that going on when I visited the site in January, but it was mostly contained to one or two blocks. That is no longer the case:

Clearly, motorists abhor a vacuum.

The design here called for a mountable curb but no other physical separation. That's not going to be enough. To make this section of the greenway a viable bike path, Jersey barriers, heavy planters, or some other form of protection will be needed.