Changes to car design in the 1950's wouldn't account for much difference. No such thing as crumple zones back then. The things you can probably attribute most to safety improvements are the ones that don't get mentioned. For a start, more and more drivers would have had to actually pass a driving test to obtain a licence, rather than just pay their few shillings fee with no test. Then you've got improvements to road surfaces, barriers to stop vehicles crashing down hillsides, better street lighting etc. Statistically minor rural roads are the most dangerous, so it makes perfect sense that as more higher quality roads, dual carriageways, then motorways were built, the accident rate per mile travelled would start to decline. We have now pretty much got down to the level at which safety and accident rates are not going to get any better. From now on, expect the figures to more or less just bump along from year to year at a fairly constant level with a bit of apparently random variation.