Under Article 48 of the Road Traffic Safety Law Implementation Regulations, what is the right-of-way rule when two vehicles meet on a narrow mountain slope?

1. The uphill vehicle has priority; however, if the downhill vehicle is already midway down and the uphill vehicle has not started up, the downhill vehicle has priority
2. The heavier vehicle always has priority regardless of whether it is going uphill or downhill
3. The downhill vehicle always has absolute priority because gravity makes it impossible to stop
4. The vehicle carrying the higher value cargo has priority upon displaying its invoice