Please don't illuminate the road with a flashlight.
Flashlights are circularly symmetric. Car headlights are optimized for the road in front, and correctly aligned to not dazzle oncoming drivers.
A circularly symmetric light source illuminating the road WILL dazzle oncoming drivers.
Any solution to illuminate the road requires high and low beams, with the low beams being selected when there's an oncoming driver. Both beams need to be optimized for illuminating the road or you'll see little benefit and LOT of dazzling of oncoming drivers.
If modification is forbidden, your only option is to purchase another car with better headlights. It is unfortunate that there are laws that restrict what you can do, but the purpose of the laws is to prevent you from accidentally dazzling oncoming drivers. The cost of designing a custom light source far exceeds the cost of getting rid of your car and purchasing one with better headlights to replace it.
Any light source temporarily mounted to your car is a modification of the car's headlight system, and hence likely to be illegal.