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A rule created for rural US roads in the 1960s still masks speeding in Ireland in 2022

— One motorist hit 60km/h beside a speed ramp in a 30km/h zone.
— Average of 89 motorists per hour broke the 50km/h limit in Chapelizod.

An arbitrary rule called the ’85th percentile’ is being used by council officials in Ireland to mask speeding and to reassure councillors that concerns from residents are baseless when recorded speed data shows otherwise.

Basically, if a speed count is conducted and the 85th percentile is close to or higher than the speed limit, that means a significant number of motorists are likely speeding. But traffic engineers employed by councils will use the 85th percentile to basically cover up speeding.

Long-time readers will remember has previously reported on the extent of urban speeding around Ireland, but it might come as a surprise to many residents and councillors than, even in the less-car centric Dublin City Council area, 1960s thinking is being used to mask speeding.

After spotting mentions of speed surveys in a recent Traffic Advisory Group reports to councillors, filed a Freedom of Information request for the survey data, two examples on the northside and two on the southside.

The northside examples — Shandon Road in Phibsborough and Ventry Drive in Cabra — showed no speeding beyond their 30km/h speed limit in surveys that were conducted between 7am and 6pm.

One local resident near Shandon Road, who spoke to, said speeding on it and some adjoining streets is more noticeable in the evenings and at weekends — typically what appear to be delivery drivers, and food couriers.

Ventry Drive is a short street where you would not expect much speeding. The data seems to reflect this.

The two examples mentioned South Central Area Committee TAG report are quite different — Lucan Road in Chapelizod, and Thomas Moore Road in Walkinstown.

A reassuring note to councillors said: “Following the assessment carried out by the Traffic Advisory Group Area Engineer they did not observe any speeding in the area at that time of the assessment so they carried out a speed survey on the Lucan Road on Tuesday 09/12/2021 . The results of the speed survey came back with an 85th percentile reading of 47.8Km/hr in a 50Km/hr speed zone and a combined average speed of 36.7Km/hr. With this information it would not be recommended to have any traffic calming measures installed at present.”

But the Lucan Road data, provided to under FOI legislation, shows that 875 motorists were recorded going between 50-60km/h, another 62 motorists between 60-70km/h and 6 above 70km/h.

This means 9.4% of vehicles passing were speeding — or an average of 89 motorists per hour between 7am and 6pm.

Regarding Thomas Moore Road, the South Central Area TAG report last month said: “Following the assessment carried out by the Traffic Advisory Group Area Engineer it would not be recommended to add additional traffic calming to Thomas Moore Road. Ramps are currently in place and have been proven to be effective during a speed survey. The results of the speed survey returned a reading of 0.25% speed violations and the 85th percentile reading of 34km/h during the 24hr period.”

Last year, in May 2021, the same issue was also looked at by TAG, the report said: “Not recommended. After a site inspection the area engineer did not notice any speeding in the area at that time so he carried out a 24hr speed survey for Thomas Moore Road. The results of the speed survey came back with readings of 0.25% speed violation and the 85th percentile reading of only 34km/h during the 24hr period within a 50Km/hr speed limit. With this information and the fact it’s already traffic calmed with ramps it would not be recommended to have any additional traffic calming measures installed on this road.”

There’s kind of a large problem with this. Thomas Moore Road is in a 30km/h zone and has been since 2020.

The data shows that 235 motorists exceeded the speed limit within 24 hours — 26% of the total number of vehicles, and that 111 motorists were above 5km/h or more over the limit.

This is despite the speed recorder being aimed right beside a speed bump.

Despite this, 30 motorists exceeded 40km/h or more, and only one of these were recorded as a two-wheeler. The TAG report still claimed to councillors that: “Ramps are currently in place and have been proven to be effective during a speed survey.”

IMAGE: The location on Thomas Moore Road where speed data was recorded.

In the below Not Just Bikes video on speed limits, YouTube presenter Jason Slaughter, explains: “But the bigger issue with the 85th percentile is that, like most things wrong with American traffic engineering, it’s a 1960s metric designed for use on rural roads, that was then applied to everything, from highways to city streets, and everything in-between.”

He said it kind of works for high-speed connections between two places and “The problem comes when this design approach for a rural road, is applied to a street. Unlike a road, a street is not a thoroughfare, it is a destination. A place for productive city life. A street will be lined with houses or businesses. It will also have pedestrians and cyclists, who aren’t being considered at all in that 85th percentile.”

Recording the speed is slightly different than applying limits, but the same principles apply — the motorists traveling above the speed limit are still there, the problem is just masked. is reader-funded journalism. That means it's funded by readers like you.

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Cian Ginty

1 comment

  1. Thanks for highlighting the 85th percentile datum none sense about setting urban speed limits.
    I have been calling for road authorities to reveal the meta-data recorded from the speed awareness signs at every Transport SPC and at Joint Policing Committee meetings. Councillors and Gardai need to know that speeding in urban areas is rampant. They need to be reminded about the exceedences on a regular basis. The RSA annual (nothing posted since 2018!) free-speed surveys are just that, annual, and not sufficient.
    The free-flow of traffic concept is doing a lot of damage.
    The Department of Transport should direct road authorities to reveal this data on a regular basis.


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