Tesla Motors' (TSLA) stock is down 15 percent as of 2 PM ET to $25.45.

The stock of the pioneering electric vehicle firm is tumbling because the market risks being flooded with some of the 75 million shares that insiders are now free to sell.  The 180-day post IPO lock up expires today according to this 10-Q filing. As the filing states, "The market price of our common stock could decline as a result of sales of a large number of shares of our common stock in the market in the future, and the perception that these sales could occur may also depress the market price of our common stock."

Tesla shares were hit last week after Capstone Investments analyst Carter Driscoll initiated coverage of the electric car firm with a Sell rating and a $22 price target.

"Investing in Tesla is a bet that the Model S will be on time and on budget and that customer adoption is robust and consistent. A lot of strong execution has to occur to justify the current valuation and management provides limited visibility," Driscoll states. "We expect that hybrid EVs and plug-in hybrid EVs will continue to take the lion’s share of green conscious automobile customers given their lower price points, better driving ranges and availability from brand name OEMs such as General Motors, Nissan and Toyota.”

Driscoll also believes that Tesla’s competitive advantage in battery technology is likely to decline as key OEM partners become competitors.  “We acknowledge [Tesla's] battery packs are among the best and lowest cost in the industry today,” he states. “Longer-term we are skeptical Tesla can drive the next leg of growth as they must convince OEMs to utilize their powertrain technology as well as battery packs.”

The market might be jittery from the threat of insider sales, not actual sales, but certainly the market is at risk of being overrun with Tesla shares. 

The leading stories in electric vehicles this year have been Tesla's success and the roll-out of the Chevy Volt.  Other 2010 highlights in transportation markets are reviewed here.