As others have alluded to, the coax tends to limit the maximum travel of a magnetic base should it break loose, so any actual damage is likely to be limited to your own vehicle unless there is a lot of free and unsecured coax that would allow the antenna and base to travel a significant distance. In practice they do not typically come loose if properly installed and an adequate magnetic base is chosen for a given sized antenna and vehicle mount location. And if they do come loose, the coax nearly always keeps the antenna closely associated with your vehicle (up to and including the opportunity to reattach elsewhere) so long as there are no long lengths of unsecured coax (so try to limit your unsecured coax lengths). If one were to totally separate (if somehow even the coax detached) the real danger would not so much be to the driver behind you (aside from their possible loss of control due to unexpected surprise) as the antenna and following driver are likely moving at similar velocities along the same vector (its time on your vehicle imparted the forward momentum), so any impact is much more likely to be frightening to the other driver than it is to inflict major damage. What would be genuinely terrifying in this unlikely case (with coax detaching entirely) is if the antenna struck an oncoming vehicle as then the forward momentum your vehicle imparted on the antenna/base will be added to the forward velocity of the oncoming driver! However, even if such penetrates the windshield it will not impart its full force onto the other driver/passenger as considerable energy would have been consumed in the act of penetrating the windshield (windshields are rather tough), but that does not mean that there is not a chance of serious injury -- and that is excluding the chance of serious injury if the shock and surprise causes the other driver to lose control of their vehicle.