Scale 1: 250,000 literally means for 1cm you calculated on the map it is magnified by 250,000 times. So if that road is 4-6cm, you just multiply it by 250,000 times and you get 1,150,000 cm.
Since you wanted to be in Km. the 100 cm is 1 meter and 1000 meters is 1 km. Therefore, 1,150,000/ 100 x 1000 = 11.5 km . Answer: 11.5 km.
