Sadiq Khan has been criticised for an "eye-watering" £859,000 budget for adverts around Transport for London's 20mph zones.
According to a Freedom of Information Request, TfL has spent £514,000 of the budget so far, with just £345,000 left.
Responding to the figures, Conservative mayoral candidate Susan Hall accused Khan of "wasting" taxpayers money on "ads promoting himself".
She said: "This is a huge waste of money promoting a policy that is blocking up our main roads.
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“Sadiq Khan needs to be honest with Londoners about how much taxpayers' money he is wasting on ads promoting himself, including some that appear to be designed to benefit his election campaign.
“As Mayor, I will put Londoners first and remove 20mph limits on main routes like Finchley Road."
Meanwhile, Kieran Terry, Conservative Party Candidate for Greenwich & Lewisham (London Assembly) said: "When Sadiq Khan has hiked his council tax by 9.7 per cent a lot of Londoners will be very concerned to see this level of spending on one propaganda campaign.
"This eye-watering figure is clearly far more than what you would need to spend to notify road users about a new speed limit on their street.
"We are told time and time again by Sadiq Khan that he can’t afford to pay for basic things but it appears he can find the money when it comes to spending it promoting himself."
But a spokesperson for TfL said the "spending on communication is a key part of our overall investment in reducing road danger."
Last month, TfL announced it is lowering the speed limit to 20 mph on a further 65km of London roads throughout the rest of this year.
The latest rollout represents the biggest expansion of 20mph zones yet and is part of a wider reduction in speed limits across 140km of London's roads.
It covers roads in seven boroughs across London - Greenwich, Kensington and Chelsea, Lewisham, Southwark, Wandsworth, Merton, Bromley and Lambeth.
The rollout - which began throughout the central congestion zone during the pandemic - is aimed to be completed by May 2024.
TfL said lower speed limits play a "critical role in the Mayor’s Vision Zero plan to eliminate deaths and serious injury on the transport network".
But hitting out at the 20mph zones, Ian Taylor, a director at the Alliance of British Drivers, told The Telegraph: "The war on motorists, which politicians keep promising to end, is in fact intensifying.
"There may be a few places with 20mph justified, such as outside schools, not A-roads, but the current Mayor of London seems determined, bit by bit, to turn virtually the whole place into 20mph maximum areas.
Conservative mayoral candidate Susan Hall accused Khan of "wasting" taxpayers money on "ads promoting himself"
"That doesn’t necessarily surprise us because he does so many things that could be regarded as anti-car. The trouble is it’s getting to the stage where you’ll get cycles and scooters going as fast or faster than the cars, which could have problems of its own."
A TfL spokesperson said: “Keeping Londoners safe as they navigate our city is TfL’s top priority and 20mph speed limits save lives. Collision data from around the world shows that the faster a vehicle is travelling, the more likely it is that a collision will occur and reducing vehicle speeds is the single most important factor behind the likelihood of a collision occurring and the severity of injuries.
“It’s vital that we make people aware of new speed limits, which is why we launched a multi-channel communications campaign targeting drivers to raise awareness. This spending on communication is a key part of our overall investment in reducing road danger.”