A basic answer is this; Provided the car park does not come under Highways law; which in this case it probably does not as its not open to the public etc. then all things may apply inclusive of RIDDOR e.g. if 2 vehicles collide whilst they are at work then it is not an RTC under highways law but it is a vehicle crash nevertheless and RIDDOR needs to be evaluated and accounting for as if they collided because the lighting was too poor to drive safely then it may become the employers fault/part fault. And as U control the area U have the enjoyment, duties and liabilities that come along with that control Each case would need evaluating under its own circumstance against the parameters of RIDDOR etc. Additionally evaluate the T&C's relating to your use of the car park for the needs of the work noted as other conditions may also apply best of luck