(Reuters) - Comma.ai is canceling development of a planned self-driving device after U.S. safety regulators threatened to prohibit its sale, the San Francisco-based startup company said Friday on Twitter.<div><a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=fZlohec1hnY:oh7W63IpGu4:yIl2AUoC8zA"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?d=yIl2AUoC8zA" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=fZlohec1hnY:oh7W63IpGu4:V_sGLiPBpWU"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=fZlohec1hnY:oh7W63IpGu4:V_sGLiPBpWU" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=fZlohec1hnY:oh7W63IpGu4:-BTjWOF_DHI"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=fZlohec1hnY:oh7W63IpGu4:-BTjWOF_DHI" border="0" /></a></div><img src="http://feeds.feedburner.com/~r/reuters/technologyNews/~4/fZlohec1hnY" height="1" width="1" alt="" />