Suggested

Additional role of Doctor

We know well that a doctor role on the benchmark is to reduce health loss when training drivers. I think in my mind is to add the role of the doctor rather than just what i said. It is to help drivers regenerates health gain (normally 5%/hours).

Or daily additional refill based on doctor level. For example:
If doctor level 1-10, doctor level, max 10. 
If doctor level 11-20, half or doctor for above 10, max 15.

For example, if level 16, refill will be =(10)+(16-10)/2=13

Thanks for replying this message. You are fairly right!