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 Coll & Tiree 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 on the island. After around half a mile, the old pier at the southern edge of Arinagour is found, which is the road's original terminus. We then pass the island post office before reaching a fork. In the past, all arms at this junction had Class II status. Our road forked left as it still does but the right fork was the B8071; it had a spur to the island hotel before rejoining our road just before the B8071 now heads off northwards.
The B8070 again turns left at what is now the only junction with the B8071. It 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 causeways have been built. Turning westwards for a while, a scattering of farms and houses sit back from the road, and then fields are found once more, with the island's airfield off to the right. The terminal building sits on the roadside; it is a medium-sized timber building with a dozen car parking spaces outside.
A sharp left-hand bend is followed by a gentler right-hander, after which the B8070 comes to an end at a junction with a farm drive. The unclassified road ahead continues for some distance, to end in a grassy car-parking area, with a track leading down to the beach. This track continues along the beach behind Crossapol Bay to reach Crossapol House at the western end of the island.