That doesn't sound right... seatbelt laws forced people who otherwise would have felt confident enough to drive without it to wear one. Low confidence drivers always had the option to wear one at any time. From only study I could find on the subject [1]:
"We distinguish, following the literature, between fatalities among car occupants, who may be directly affected by
using seat belts, and fatalities among nonoccupants (pedestrians, bicyclists, and motorcyclists), who do not use seat belts and can thus be affected by seat belt use only indirectly... Our findings indicate that seat belt use significantly reduces fatalities among car occupants, but does not appear to
have any statistically significant effect on fatalities among
non-occupants. Thus, we do not find significant evidence for
compensating behavior."
Maybe the invention of seat belts could have that effect, but that coincided with so many other changes to automobile technology / urban development that any observed effects are correlated at best.
"We distinguish, following the literature, between fatalities among car occupants, who may be directly affected by using seat belts, and fatalities among nonoccupants (pedestrians, bicyclists, and motorcyclists), who do not use seat belts and can thus be affected by seat belt use only indirectly... Our findings indicate that seat belt use significantly reduces fatalities among car occupants, but does not appear to have any statistically significant effect on fatalities among non-occupants. Thus, we do not find significant evidence for compensating behavior."
Maybe the invention of seat belts could have that effect, but that coincided with so many other changes to automobile technology / urban development that any observed effects are correlated at best.
[1] https://web.stanford.edu/~leinav/pubs/RESTAT2003.pdf