One reason why people take no notice of the
‘PLEASE CLEAR IT UP‘
signs is because the signs themselves are small and inconspicuous. If the signs were 3-4 feet across and included ‘£1000 PENALTY’ errant dog owners may feel obliged to comply.
The same would apply to speeding boats. The signs ask boaters to slow down in relatively small writing rather than telling them in big letters. By the time a boater reads this sign he has already passed about 15 moored boats on official moorings. It is actually set lower than the roofline of a narrow boat.
No sign has any effect unless a. it can be easily seen b. it is properly positioned c. in the case of these two signs they include the penalty.