The Alliance of British Drivers (ABD), which opposed the proposals at a Scottish Parliament committee last week, responded to a taunt about speeding drivers by tweeting "f*** of (sic) a*****e".
It was sent from the official Twitter account of the group, which describes itself as "the UK's leading campaign group for drivers who can think for themselves".
The tweet appears to have since been deleted.
It was posted as part of a Twitter conversation about evidence given on behalf of the ABD and by other groups to Holyrood's rural economy and connectivity committee last Wednesday.
Road safety researcher Eric Bridgstock, speaking on behalf of the ABD, had claimed to MSPs that cutting speeds on residential streets could increase casualties by lulling pedestrians into a "false sense of security".
@JustThinkOXON tweeted: "Anyone claiming that higher speeds increase safety is either a liar, an idiot, or both."
Nigel Pain replied: "An ex-colleague of mine used to say that he was safer at higher speeds because he was more alert."
That prompted the ABD to tweet: "Patently obvious to anyone with their brain switched on."
Alex Quayle, of cycling and walking group Sustrans Scotland, replied: "Ha ha ha. You: I am so smart and capable. My brain doesn't need speed limits to keep me safe. Also you: going slow makes me drive into stuff more often."
Hours later, the ABD tweeted back: "F*** of a*****e!".
The Green MSP promoting a members bill to introduce the lower limits was dismayed at the tweet
Mid Scotland and Fife MSP Mark Ruskell said: “The level of argument coming from some of those who oppose my 20mph Safer Streets bill is frankly embarrassing.
“It should be obvious to anyone who’s engaged in this debate that ill-informed, reactionary groups like the ABD, whose response to fair challenge is to insult or block, have zero credibility.
"Leading academics and road safety campaigners can point to scores of evidence that reducing speed cuts accidents, saves lives and promotes cleaner travel.”
Mr Quayle said: “I don’t imagine many people listen to the ABD and I’m not about to start simply because they resort to abusive language.”
ABD director Paul Biggs said: “The Alliance of British Drivers are dismayed to learn about recent inappropriate, unauthorised tweets by an individual from one of our Twitter accounts.
“We only found out via a Google Alert three weeks after the incident, which linked to an article in The Scotsman.
“It appears the tweets were subsequently deleted by the individual.
“We apologise to everyone involved as this isn’t an image that we want to project, nor is it representative of the ABD.
“The incident undermines the evidence-based representations that we make to policy makers.
“We’d like to thank The Scotsman for drawing our attention to the issue and we understand they were unsuccessful in their attempt to contact us directly.”