Is it permissible for a woman to travel by car with a driver while her brother is in another car?

It is forbidden for a woman to travel without a mahram (definition), just as it is forbidden for her to ride alone with a non-mahram driver, because this constitutes unlawful khalwa (definition) (private seclusion). However, if there are other women with her, thus eliminating khalwa, and her mahram is present in another car traveling at the same speed alongside her, then there is no harm in that, provided the driver ensures he does not fall behind the car containing the male mahrams.