this may have been an option pre 9-11 but the US no longer fucks around with people who pose as a threat to the public, especially on the capitol. Also, its nearly fucking impossible to shoot the tires of a moving car, not to mention the risk you put on bystanders if you miss.
you seem tripped up on the fact that she was a depressed, postpartum mother. would your opinion have changed if this was a man driving the car?