The B8070 is the longest of the three roads on the Isle of Coll. It was originally unclassified but gained its number around 1930, at the same time as the other two roads. It runs from the ferry pier at Arinagour north to the junction with the B8071 and then south west to serve the south of the island.
From the pier, the road runs north on a new road, built when the new pier was constructed in deeper water allowing the ferry to berth at the island, rather than anchoring in the bay and being served by 'flit boats'. After around half a mile, the old pier at the southern edge of Arinagour is found, which is the road's original terminus. The route then passes the island post office before reaching a fork. In the past, all arms at this junction had Class II status, the B8070 forked left as it still does, while the right fork was the B8071. This spur to the island's hotel soon rejoins the B8070, but is now unclassified. A short distance further on, the B8070 again forks left at what is now the only junction with the B8071.
The road passes the church and starts its journey across the south of this pretty island. A few fields are passed before the road heads out into open moorland, winding round the contours above Loch Airigh and heading roughly south past Loch na Cloiche, before skirting the shore of Loch Ronard so closely that a couple of short causeways have been built. Turning westwards for a while, a scattering of farms and houses sit back from the road, forming the settlement of Acha, where the road briefly crosses the 40m contour. Then, after winding round some bends which cross a section of rocky terrain, fields are found once more at the settlement of Uig. Here the island's airfield lies off to the right, with the terminal building sitting on the roadside; it is a small timber building with a dozen car parking spaces outside.
At the end of the straight past the airfield, the road reaches a T junction, where it turns sharp left. This is the only real junction on the route, with the side road heading north to a car park, with a track continuing through the dunes to the B8071. After a short straight, the road turns back to the west and the B8070 comes to an end at a junction with a farm drive just after a cattle grid. The unclassified road ahead continues for some distance, to end in a grassy car-parking area, with a track leading down to the beach. Another track continues through the dunes behind Crossapol Bay to reach Crossapol House at the western end of the island.
