Kevin Hughes said it would take less than a second to ride over the marks
Council officials have put the brakes on claims road markings are for "the shortest cycling lane in Britain".
The 2.5m-long markings, with a cyclist logo in the middle, begin and end in Adam Street, Adamsdown, Cardiff.
They were spotted by keen cyclist Kevin Hughes who posted a photo on the forum of Cardiff Ajax Cycling Club.
Cardiff Council denied it was a cycle lane but said the markings were too wide and would change to indicate a turning as part of a contraflow.
Mr Hughes saw the markings near Cardiff Prison as he was driving home one night.
He calculated a cyclist travelling at 12kmh (7.5mph) would take less than one second to pass over the markings.
He said it looked like a cycle lane that was "probably the shortest one in Cardiff may even the shortest one in Britain".
Cardiff council said the markings were too wide and would have to be reduced by around a half their current width to conform with the plans for them.
A spokesman said: "The road marking in the photograph forms part of a new contra flow cycling facility extending between Adam Street and Churchill Way via Sandon Street and Guildford Crescent.
"It will be particularly useful for cyclists accessing the city centre from the Splott and Adamsdown areas.
"The marking is intended to highlight the point at which cyclists can turn left off the carriageway to join the contra flow facility. Accompanying signage is soon to be installed.
"Original plans for the road marking have not been followed and it will shortly be reduced in size."