I've asked my T, overtime or not, to plan for a few minutes at the end to ground myself before I go out into the world again. Like you said, it's just not safe to hop in the car and go floating off when you're still feeling rough.

A few minutes before we end, she will gently check in and ask me how I'm feeling, and will usually run me through a quick somatic grounding--deep slow breaths, feeling my body in the chair, connecting to the floor with my feet, etc. That usually helps bring me into myself so I'm good to leave. Maybe you can ask your T to be in charge of the clock (her job) and plan for those few minutes to make sure you're safe before you go.