I suggest adding Crash System meanwhile you are running fast to Deliver the Client you always Hit a tree or your Taxi Get damage often while Jumping from hill to deliver fast, And your Timer will be constant without any changes so We can decrease Timer if your car got damaged a lot.
I suggest also addition of stunt system that gives you bouns to payment like crazy Taxi game or GTA v
as you doing some stunts at different ramps or aviable places to jump from it
Reduced taxi driver level up requirements by 15%. (Arran + Sinon)