People also simply drove less during that time. Night spots that used to be busy 'til 1:00 in the morning suddenly were closing at 8:00 or 9:00 pm. The town I lived in "rolled up the sidewalks" at 8. Kids used to cruise up an down Main Street 'til late night and then suddenly the town was dead after sunset. The oil embargo made a huge impact on our lifestyle that went way beyond driving slower.
I believe there was also a sudden lowering of highway deaths the year they reduced the time that the death occurred after the accident from one year to one month. Sort of a statute of limitations on cause of death. The actual death rate didn't drop but what counted as a car accident death did.
Methodology can make a big difference in statistics, so if some other country boasts of a low suicide or murder rate, one should immediately ask "what counts as a murder or suicide there?"