LA is actually four or five times larger than Melbourne (18.8 million). I suspect you're referring only to its inner city population, which is ~4 million. We count our urban population differently in Australia than the US does. If we take the number of pedestrian fatalities in greater Melbourne over five years (274/2=137, say) and multiply by 4, to bring it into proportion to greater LA, you get a figure (548) that is actually higher than LA...
Huh. I learnt something new today! I guess I need to improve my googling skills. A search for 'LA population' returned only the inner city figure. Again though, the numbers are making my head hurt. That means that the population of greater LA is almost 3x the population of Victoria. Squashed into 1/3 of the space. I guess we have a lot of empty space, huh?