on the M25 ACW on Saturday early evening (still daylight), about to leave onto the M3. Plenty of traffic but flowing nicely at about 70mph. For those who know this junction, there are four lanes of continuing M25 and two lanes coming off to the M3. We were approaching the point of separation (in the right hand of the two exiting lanes, ready for heading down the M3 away from London) when we saw the car in front put his brakes on. Mrs PXW was driving so I had the view down the inside where I saw a white Prius, stationary, straddling the white line in the middle of the two M3 lanes. Emergency stops all round (and thankfully we all managed to avoid a collision) then the Prius indicates right, makes its way across the intersection and the start of the M25 hard shoulder and continues down the M25. Clearly went the wrong way and realised too late. But rather than just continuing and correcting course at the next junction, they decided to endanger themselves and everyone around them by just stopping in the middle of the motorway. If ever we needed a police patrol car to come along, it would have been then!