Car crashes are still responsible for a million deaths and something like a trillion dollars in damages every year. Reducing the number of people on the road with slowed reaction times would only have to make a very small relative dent in that to have a very large absolute effect.
On the other hand, toxoplasmosis treatment is almost certainly not the low-hanging fruit here - better cultural attitudes and vehicle safety features already make driving much safer in some places than in others, and self-driving cars may make the very concept of driver reaction time moot in a generation.
Treatment on a large scale presents different concerns than on a small scale.
Toxoplasmodi gondii is a parasitic protozoa who's primary host is cats but also infects other mammals, primarily mice and rats but including humans, as part of its life cycle. Infection by Toxoplasmodi gondii is called Toxoplasmosis and may be acute (flu like symptoms) or latent.
Toxoplasmosis is extremely common. Worldwide, about 30% (US 11%; France 88%!) of people about of people have Toxoplasmosis.
Toxoplasmosis is known to cause behavioral changes in rats:
Observational studies suggest that latent Toxoplasmosis may also cause behavioral changes in humans (source paper). The observed differences between infected people and non-infected people include: