Police say speed was a possible factor in a single-car crash on the H-2 freeway which injured two people -- one of them critically.  It happened at 7:13 Thursday night.  A man in his 20s was driving a Mazda sedan northbound, when he decided to take the Ka Uka offramp - too late.  The car crashed into a drainage ditch, then hit a guardrail, rolled over and slid more than 100 feet on its roof.  The man was taken to a trauma center in critical condition with head and internal injuries.  His 22-year-old female passenger was also hospitalized in serious condition.