TransConsult experts explained their choice of route from four options and estimated the cost of a difficult construction at 219.5 million baht.
But a meeting chaired by Phuket Governor Maitree Intrusud at Phuket Provincial Hall in Phuket City also heard the route would have negatives as well as positives.
Carving the road up to 250 metres across the Nakkerd Hills between the Luang Pu Supa Temple in Chalong and Harsipbee Road in Patong would save commuters and tourists up to 30 minutes in travelling time.
At present, the choices are much longer routes through Chalong Circle via Kata-Karon or through Phuket City.
But a quality roadway, with a three-lane passing section at the crest of the hill, good lighting and concrete buttresses to prevent landslips would also have environmental and social effects, the meeting heard.
A total of 117 different kinds of protected animals, including four at-risk species, made their homes in the Phuket hills, the governor was told.
Road building through the natural area would create pollution and permanent habitat divisions through a wild, natural area.
Development was likely along the roadway and there would also be unknown social consequences in opening up Phuket's south to rapid transport from Phuket's west coast.
About 20 percent of residents polled in both Chalong and Patong opposed the roadway, according to a TransConsult survey.
One of Governor Maitree's predecessors, Governor Wichai Praisa-ngob, was keen to establish the route as well as a second highway between Phuket City and the airport to solve Phuket's traffic woes.
Talks have been going on since 2010.
A Roads Department representative suggested yesterday that Governor Maitree sign the paperwork for presentation to Cabinet. Governor Maitree felt it was a project that the Roads Department should support.
TransConsult Executive Director Banpote Jaroensathayatam, Engineering Department Manager Anon Sakburanaphet and Environmental Specialist Rungrtip Krutnoi attended yesterday's meeting.