Making vehicle on trips mandatory

When recording mileage, the Vehicle field is often left blank, resulting in trips being booked on the wrong car. This causes errors in mileage reimbursement. This is because employees who drive a business car receive a different (lower or no) reimbursement than when they drive their own car. By making the Vehicle field mandatory (or defaulting to 'own car') this can be prevented and the registration and reimbursement remains correct.

The filter business/private is already used, so an additional differentiation needs to be added. An employee can register a business trip, with a private vehicle.