"All-lane running" has been introduced between junction 10a for Wolverhampton and junction 13 for Stafford.
When trialled on the M25 near London, it "reduced congestion and led to more reliable journeys", said Highways England.
Lay-bys are replaced by emergency refuge areas - but some organisations say there are not enough of them.
The Central Motorway Police Group said the lack of lay-bys was "cause for concern" and there was "minimal scrutiny" of the scheme on the M6.
Edmund King, president of the AA said: "Laybys are every two and half kilometres - that is about one and a half miles.
"If you can't see a lay-by you have to stop in a live lane of traffic and that is so dangerous."
Matt Dallaway from the RAC said all-lane running as a concept was fine but he was concerned safety had not been tested.
MP's on the Commons Transport Committee have launched an inquiry.
Highways England said: "Our motorways are some of the safest in the world and the evidence shows collision and casualty rates on the M25 are down."
"As we gradually roll out these upgrades on other motorways we will continue to work closely with all the emergency services so we can ensure safety is maintained."
All-lane running is currently being installed on the M5 between junctions 4a for Bromsgrove and 6 Worcestershire North.

Fishing and other activities on its waters are credited with supporting some 513 jobs.
The last review of the economic impact of the river was carried out in 2006.
The latest report by consultants SQW found that despite "a couple of testing years for anglers" its annual impact had risen from £18m to £24m.
The River Tweed Commission (RTC), which ordered the report, welcomed its findings.
Chairman Douglas Dobie said that the economic benefits of the river allowed them to invest in scientific research to help manage the river.
He said that in the past decade the RTC had channelled research grants totalling £1.3m into the Tweed Foundation.
"This report underlines the success and importance of managing the River Tweed on a long-term basis," he said.
"Over the past decade the economic contribution of the activities on the Tweed has continued to grow, as has our scientific understanding of the whole river system.
"We are unique in Scotland in this regard."
Mr Dobie said the report underlined the "increasing importance" of the river to economies north and south of the border.
"With the commitment of the commission to keep investing in the management and science of the river through the Tweed Foundation, we can be confident that the positive impact will continue to be enjoyed locally, nationally and internationally," he added.
A scheme allowing motorists to drive on the hard shoulder of the M6 is "dangerous," motoring groups have said.
[NEXT_CONCEPT]
A report has found the value of the River Tweed to the economy of the Borders and North Northumberland has risen to £24m-a-year.