Johno Smith Single Line Work Positioning on the Unicender.
In a word, SAFETY! NZ Arbor BPG Committee member Mark Roberts got it right when he said "we need to get a few key safety points into existence before someone kills themselves".
The BPG Committee is well aware that SRT Ascent systems are becoming more and more commonplace in NZ tree work.
The committee is probably also aware that real-world application of these ascent systems suggest they are not as safe as first imagined. The problem is compounded when the systems are mis-configured or mis-used - which is very easily done.
In theory at least, SRT Accent systems do present a compelling safety argument - possibly the reason why this method is endorsed by the ITCC and why most of the major arb companies in New Zealand use a single line for ascent into big trees.
Efficiency, lower-ability of the climber in the case of an emergency and the setting of an additional lowerable, access line in the tree all sound very safe indeed.
The problem is, work-place reality is slightly different.
Climbers regularly Work Position on SRT Ascent equipment and the toothed-cam ascenders commonly used in SRT Ascent are not designed for that application (as cavers well know).
Moving laterally on the single line (using toothed-cam ascenders), stepping up onto a branch, or introducing even small amounts of slack into the SRT Ascent system can be fatal.
It's safety concerns like these that are driving many arb companies and individual aerial arborists to adopt the use of Single Line Work Positioning (contrary to what some industry pundits have to say).
Single Line Work Positioning equipment allows for efficient ascent as well as lateral movement in the tree, not unlike DbRT - in short, the best of both worlds in terms of safety. A separate access line can still be installed if that was deemed necessary.