There’s a lot to be said for writing by hand: It’s therapeutic much of the time. And it allows the ideas to flow. This is my preferred method for writing fiction. Non-fiction tends to work best if entered directly into a text document (not Word).
A major attraction for writing by hand is the correlation between physical movement and mental gymnastics. Hand, eye, and brain co-ordination. This can be challenging even if the writing surface is (relatively) sturdy and stable. The environment is the biggest impact here.
Now, add: Bumpy uneven roads, traffic; vehicle suspension that is demonstrating a distinct need for replacement; and an experienced drive whose idea of braking and moving is to stamp the pedal then jerk the vehicle with the gas pedal each time.
While hand/eye/brain co-ordination still works, writing in a moving vehicle is unpredictable: Legible cursive becomes semi-legible, and worsens if you’re falling asleep. Still, the challenge is fun, if faced only once in a long time! 🙂