The series is named for a David Bowie song whose lyrics closely match the plot of the show.
It would also be a reference to Sam waking up in 1973 and as he puts it, it's like "landing on another planet" and the series focuses on Sam's attempts to acclimatise to 1970s Britain whilst still trying to get back home; the show is all about Sam's Life on Mars.
In Ashes to Ashes (2008)the Pierrot Clown represents death to Alex - it looks like her father, who killed her mother and himself in a car bombing, and is dressed like David Bowie from the "Ashes to Ashes" music video, the song for which was playing in the car when it blew up. So why does the Test-Card Girl represent death for Sam? Because he's currently lying in a hospital bed connected to an EEG. If it starts to flatline - the same noise the TV makes when the Test-Card Girl is on - then it means he's died.
Much of the series becomes Fridge Horror after the Ashes to Ashes (2008) finale. Gene's world is a copper's purgatory, they have to 'finish the job' to pass on, and Gene's role is to help them. It's implied that any copper who dies in that world without finishing the job goes to hell. Now watch the ending of the Life on Mars episode with Harry Woolf again. Fridge Brilliance if applied to Glen, but Harry is dying and has crossed the pavement. This man is Gene's mentor.
From the same episode, there's a throwaway line in which Gene claims part of his motivation for nabbing Harry's nemesis is that he's seen good coppers die trying to settle old scores. Presumably, he knows what he's trying to keep Harry out of.
Given the revelations of Ashes to Ashes, Sam did not prevent any future murders based on what he did to Tony Crane. Not only that, but his father did murder Annie Cartwright, and is the reason she is even in this world.