The trunk road drops from 150m (492ft) to 20m (65ft) as it enters a valley at the Berriedale Braes.
There is also a hairpin bend which lorry drivers can have difficulty negotiating.
Rob Gibson, SNP MSP for Caithness, Sutherland and Ross, has welcomed planned improvements at Berriedale.
He said upgrading the stretch of road had been spoken about for 40 years.
"The issue of Berriedale Braes is one that has continually been raised with me," he said.
"I have, along with many others from the county, lobbied the Scottish government and the transport minister over making improvements to the Berriedale Braes."
The MSP added: "Realigning the Berriedale Braes will be a tricky job so it is important that every aspect of the process is carried out rigorously and in great detail."
The A9 is Scotland's longest trunk road.
The Scottish government has committed to upgrading the A9 between Perth and Inverness to dual carriageway.
Transport Scotland said the contract was for ground investigation (GI) work and formed part of the preparations for the upgrade of the Berriedale Braes.
A spokesperson said: "This GI work will include the sinking of boreholes both in soil and rock as well as in situ and laboratory tests, leading to the production of reports which will help inform the work to identify a possible solution to this stretch of the northernmost part of the A9.
"We hope to appoint a contractor for this work in September.
"We are determined to work with our partners to ensure that improvements to this strategic route through the Highlands are progressed as a matter of urgency."
Transport Scotland has put a contract out to tender for work preparing for the upgrading a notorious stretch of the A9 in Caithness.