I wish to express my sincere condolences on the recent loss of your father.
But the short answer to your question is that this sequence of numbers probably only has a meaning to your own unconscious mind, and nobody else's.
The mind has an inbuilt organising function in an attempt to make some sense of what is happening around us, and often artificially associates two events with one another because it cannot work out any other way of explaining what on the surface appears to be merely a strange coincidence.
Do some personal research on Carl Jung's concept of SYNCHRONICITY
Nobody should suggest that there is some cause and effect relationship or connection between the date of your father's tragic death and the 818 appearing repeatedly from then on, but it does make you wonder if something mysterious is going on behind the workings of the Universe, when this sort of thing happens.
Numerologists generally reduce multiple digit numbers to a single digit with the exception of the three master numbers (11, 22 and 33) when interpreting it's meaning.
According to Chinese numerology, the pronunciation of the number eight sounds a lot like the word that means fortune (positive or negative), but I feel that it would be disrespectful to the memory of your late father to suggest on the basis of what is probably an unfortunate coincidence that he was somehow destined to pass over on that date, and that 818 repeating over and over again only confirms this. It was only a tragic accident, and your father's role in all this was that he he just happened to be in the wrong place at the wrong time when the drunk driver lost control of his own vehicle.
I do feel that trying to interpret the hidden meanings behind certain number sequences can often be taken too far when we resort only to blind superstition alone in order to guide us through our individual lives. Is working out the most likely meaning behind a repeating 818 going to in some way help you to grieve for your Dad? If not, then I would respectfully suggest that you should forget completely about attempting to find a hidden meaning for 818, and conserve your energies for much more practical and immediate matters than this one.