Laskern, Beruria was being machmir. She also apparently did not like Gallileans as she called him a “stupid Gallilean”. Rav Kook would say that by being overly machmir in one area she fell in another (Avoda Zara 18b Rashi d”h v’icha d’amrei meshum maaseh d’Beruria).
Joseph, if the car is moving fast how will the driver do something other than drive? Moreover, one never knows when one will run into a traffic jam, be stopped for some violation, etc.