Most cyclists would appreciate slightly wider on-road cycle lanes, but the highway planners in Poole may have gone too far this time.
Safety experts have raised fears that such confusing markings could force drivers to veer on to the wrong side of the road.
The lane runs up a steep hill and planners say it will encourage drivers to give more room to bikers who begin to wobble as the incline increases. It also allows fast cyclists to overtake.
But motorists say that most cyclists usually dismount and walk up the hill.
One driver said: 'It's utterly ridiculous. The cycle lane is wide enough to stage the Tour de France.'
Poole Council described the lane as 'advisory' and said cars could enter it.
Steve Dean, principal engineer, said: 'The cycle lane is designed to encourage motorists to give more room to cyclists as they tackle the steepest section of Constitution Hill.
'Vehicles travelling up Constitution Hill are able to straddle the cycle lane as the markings are advisory and as such are there primarily to act as a reminder to motorists to give space to cyclists.
'The lane has been marked in accordance with national guidance but further signs and cycle logos are scheduled to be added and we hope this will make the situation clearer for all road users.
'We are currently monitoring the use of the lane and most drivers seem to be using it appropriately.'
But Paul Watters, a spokesman for the AA, said: 'It seems very confusing and unusual.
'I suspect it has been put there because of the danger of wobbling cyclists or to allow a faster cyclist to overtake a slower one without going into traffic.
'But the average width of a car is around 6ft at least. Most drivers are sensible and would be concerned about crossing the line.