In the UK parking enforcement is normally determined by lines and signs shown in streets.

Why do I emphasize lines and signs? It's because you cannot have signs which enforce a parking restriction without lines indicating which part of the highway the parking restrictions apply. This may look simplistic but if you think about let us say a no waiting road sign the sign will specify the times or days or occasionally month of the year - however how do drivers comprehend to what span of the road the prohibition applies?

A single yellow line shows that there is a parking enforcement prohibition but that is not for 24/7. As a result in order to understand what the actual restriction is there have to be signs showing the times and days that the prohibition applies. These signs, which are called repeater signs, should be positioned every 60 metres along the lenght of the pavement (sidewalk in the USA) for the span of the single yellow line prohibition to which it applies. As you can appreciate there are places where a single yellow line would stretch for a considerable distance so there is a officially permitted allowance on the basis of which those signs are not required. This concession permits a council not to have repeater signs if there are signs, called Controlled Parking Zone signs, at the access to each street entering the zone in which single yellow lines are painted. These Controlled Parking Zone signs must show the limitation that applies to all single yellow lines in the zone.

Equally the limitations on loading must be accompanied by a sign and in this case kerb markings. These kerb markings are sometimes called chevrons otherwise "blips". A single yellow kerb mark shows that there is a loading prohibition but it does not in by itself indicate the days and times of that restriction only that it does not apply 24/7. Therefore it has to be accompanied by a sign giving the information applicable to the restriction.

Double kerb marks kerb marks indicate that there is no loading 24/7 and in spite of the fact that this is a absolute prohibition a sign indicating that prohibition is required to be located adjacent to the kerb marking.

A double yellow line in a street shows that there is a total 24/7 prohibition on parking (technically it's waiting rather than parking but everyone understands and employs the word parking). In this situation there is no prerequisite to have a sign indicating that there is a 24/7 restriction.

So to sum up for all with the exception of double yellow lines there have to be signs so the rule is in such situations is: sign but no lines your parking ticket is not enforceable - lines but no signs your parking ticket is ticket is not enforceable.

As well as yellow lines parking bays have restrictions - they are either only for residents to park or for the public at large or even sometimes a dual purpose bay which can be used by both residents and any motorist Equally there are parking bays which are restricted to particular drivers - e.g. for disabled drivers or are limited for particular purposes - for example loading only. The universal feature of all these bays is that they must have a sign to indicate the particular restriction e.g. is it for residents, disabled motorists or loading only. Moreover such signs are required to show the times and days that their use is restricted. Again the rule is if there are lines defining the parking bay then there has to be a sign specifying the nature of the prohibitions. Hence if there is no sign any parking ticket motorists collect cannot be enforced and you need to appeal.