In the UK, recent years have seen a trend of cut flowers being placed at the site of road accidents. Elsewhere in Europe, I have noticed shrines built at the side of roads, notably on precipitous bends in the high Sierras...
Logically, the cut flowers are really nonsense, because they will wither
and die in a matter of hours, which isn't perhaps the best reminder of a road traffic victim. Families will return to the spot for weeks and sometimes years (especially if more accidents occur there), tying tens or hundreds of pounds worth of cellophane-wrapped carnations around the streetlamp.
How about spending 50 pounds at the outset, and commemorate your family member with a floral gift that doesn't die right away? Multitudes of free-standing boxes are available at garden centres, barrel shapes, troughs, terracotta urns... Buy one of these, fill it with bulbs - crocuses, colchicums, daffodils, snowdrops, cyclamen, tulips - and see a blaze of colour in any or all seasons of the year. The box should add a sobering but prettifying element to the street, be it suburbia or the A40 flyover, and can be given reflective strips and protected by bollards if necessary. Any subsequent accidents at the same site, while very unfortunate, could add different bulbs to the display.
Future developments could include breeding of bulbs which reliably come up on the correct anniversary - well, ±3 weeks would be achievable (although I speak out of turn, I'm not a biologist). Points to be aware of - bulb boxes in the middle of busy intersections need particular care when gardening them - perhaps robotic weeding machines are needed to avoid irony in gardener demise.