Drivers have been left confused after Transport for London placed 20mph road markings in 30mph zones.
Painted road markings telling motorists to drive at 20mph appeared last week on Streatham High Road in south London, next to a sign showing the higher speed limit.
TfL said the speed limit on the road remained 30mph but would change to 20mph in December to reduce accidents.
Drivers have criticised the mixed messaging as “idiotic”.
Alex Mapp, a 41-year-old road user in the area, told the BBC’s Local Democracy Reporting Service: “I slowed down because of the new road markings, but then seeing that sign totally threw me off.
“It made me question if I was seeing okay. It’s ridiculous that no one spotted this when they were drawing the new signs on the road.
“I come down here regularly, but can you imagine if you didn’t know the area? It’s very confusing.”
Commuter Florence McCallum also told the BBC: “It’s so idiotic. You have drivers slowing down then speeding up and then slamming the brakes on again.
“It’s so, so confusing for everyone. Nobody knows what they’re doing. It needs sorting out soon.”
Biggest expansion of 20mph zones
A TfL spokesman said: “To reduce deaths and serious injuries on the roads, we are currently in the process of introducing 65km (40 miles) of new 20mph speed limits in Lambeth and other boroughs in south London.
“We are installing new signs and all signage will be updated before the lower speed limit comes into force.”
Sadiq Khan last month launched his biggest ever rollout of 20mph zones across the capital in what motorists have labelled the latest “war on motorists”.
The rollout is the biggest expansion of 20mph zones to date and covers arterial roads in seven central and southern boroughs – Greenwich, Kensington and Chelsea, Lewisham, Southwark, Wandsworth, Merton, Bromley and Lambeth.
TfL said that “lower speed limits play a critical role in the Mayor’s Vision Zero plan to eliminate deaths and serious injury on the transport network”.