From the news report last night, it happened on a straight, fast and flat section just after the Newquay turn off at the bottom of the descent from Carland Cross. God knows what the lorry driver was doing not to have seen them - browsing his twitter feed, chatting on his mobile, asleep at the wheel... it'll come out eventually.
Every morning during the Summer, I see End to Enders dicing with death on the eastbound A30 during rush hour, and there's really no need. A recommended route would be a good idea. Here's mine, for the first 120 miles or so:
A30 to and through Penzance sp Redruth. After Canonstown, take the estuary road through Hayle (B3301), then at the Loggans Moor Rdb follow the road through Connor Downs - Camborne (A3047) - Redruth - Chacewater - Truro (A390) - Tresillian - Ladock Valley (B3275) - Fraddon - Indian Queens. Next, take the parallel "old road" over Goss Moor through Victoria to Bodmin. From Bodmin tough the dual carriageway out as far as Launceston (some moorland sections can be used between Temple and Bolventor, but these take you further onto Bodmin Moor), and then take the old A30 through Lewdown, Okehampton, Sticklepath, Whiddon Down, Crockernwell, Tedburn St Mary to Exeter.
Or, skip the A30 altogether and from Fraddon head up the Atlantic Highway (A39) through Wadebridge, Stratton, Holsworthy, Okehampton to Exeter.