Dublin City Council, which introduced the controversial 30kph speed limit in the city centre, is set again to drive city motorists to distraction with the introduction of new speed restrictions.

The country's biggest local authority is now planning variable speed limits that change according to the day and time of year.

The new limits, which will force traffic to reduce their speed from 50kph to just 30kph at certain times, are on trial outside two schools in Dublin's city centre.

However, some in Dublin City Council have predicted chaos with the new system and claim they could become either a trap for gardaí to catch speeding motorists or could be completely ignored.

The new measures are to be introduced on a trial basis at locations adjacent to the Long Mile Road and the Chapelizod Road in Dublin.

One of them is on the busy Long Mile Road – on both sides of the carriageway – at an area near Walkinstown Parade and Slievebloom Park. It will apply from 8.15am to 9.30am and again from 12.45pm to 4.15pm.

The 30kph "special periodic speed limit" will apply only from Monday to Friday and will be stopped during periods of school holidays.

The second zone is a nearly 150-metre long portion of road on the Chapelizod Road and will apply from 8.15am to 9.30am and 1pm to 3pm.

A city council source said: "It is hard enough to get people to stick to one limit without changing it half way through the day. The thinking behind it is obviously correct in making schools safer but they should just stick to a single limit.

"If you start changing it during the day, you are only making life difficult for motorists and creating a trap where people can be caught speeding without realising they are doing something wrong."

A copy of the by-laws prepared for the council said: "Thirty kilometres per hour shall be the special speed limit for mechanically propelled vehicles on each of the roads specified... during the dates and times specified."

The city council has said they will be considering submissions on the new speed limits if they are received by the beginning of July.

It is thought the scheme could be further extended to other areas of the city centre area if it proves successful when school resumes in September.

The introduction of new slower limits of 30kph in the core of the city centre has already caused considerable controversy for Dublin City Council.

The limits, introduced earlier this year, are now little remarked upon, however, not least because motorists hardly observe them and they are thought to be even less frequently enforced by gardaí.

Asked how many motorists had been caught exceeding the limit in Dublin's city centre since the change, the Garda Press Office could not give an answer. A spokesman said: "We are not in a position to provide those statistics at this time."

A survey by the Sunday Tribune in March discovered that however many speed checks the gardaí were carrying out, it certainly was not enough to change driver behaviour.

In all, 97% of traffic surveyed driving in a sample of areas was in breach of the 30kph limit and 12% of motorists even managed to break the old speed limit of 50kph.