Michael McAleer answers your queries
From BP Byrne, Blackrock:
Some time ago, as a result of seeing cars in an intersection while the light was red, I made enquiries with the Garda and Dublin Corporation. The former said that they prosecute persons running red lights, the latter informed me that it had been told to keep the length of the amber period to three seconds.
The calculation of the time required to cross an intersection at any particular speed should include the length of the path through the intersection, the average length of the car, plus the safe stopping distance before entering the intersection (to avoid being rear-ended).
From personal experience, and also from calculation, I find that it's perfectly possible for a law-abiding driver to be in the intersection of Booterstown Avenue and the Stillorgan Road when the light is red.
Could you find out whether the exalted intend to set the amber period to suit the intersection and the allowed traffic speed? Otherwise the State and the insurance industry seem likely to make a packet on the resulting fines and penalty points.
According to Dublin City Council's Tim O'Sullivan, executive manager in the Office of the Director of Traffic, the real measurement of the time given to cross an intersection is not the time of the amber light but what is called the inter-green period.
This is the time between green lights and includes amber and all-red sequences.
The amber is fixed at three seconds but the total clearance is varied by extending the all-red sequence (when all lights show red). This timeframe is based on width of junction and other measurements. "So, for example, a compact junction might have inter-green time of 5 seconds, which can extend to 10 seconds in larger junctions."
According to O'Sullivan, motorists should remember that an amber light actually means "stop unless it's unsafe to do so".
As for pedestrian crossings, O'Sullivan informs us that the length of amber man here is based on a walking speed of 4ft per second, a relatively slow walking speed.
From M Dalton:
I have driven in motorways all over Europe and none is more confusing than the M50 toll bridge on route to the airport. Why is the "No Change Given" sign almost impossible to read at any reasonable distance? One finds oneself entering the lane before realising that change will not be given.
Given the excess money generated by the accumulated "No Change", would it not be possible to have clearer indication during days and nights? Is this just another form of "sharp practice" that we have to put up with, a handy little earner for the NRA or whoever stands to benefit?
Along with your signage problem at the Westlink tollbooth on the M50, we also have a gripe about the warning sign for the Eazypass booth. Eazypass operates in the outer lane and an electronic message board some distance up the road informs motorists about this.
However, we have rarely crossed the bridge without having to come to a standstill as motorists arriving at the toll, fumbling with change, notice they are in the wrong lane at the last minute, and then stop and indicate to get into another lane. This manoeuvring brings at least two lanes to a standstill and in some instances defeats the purpose of the swift EazyPass system.
From David Walsh, Co Wicklow:
I'm not familiar with Irish legal requirements, but when I was a research engineer with British Motor Corporation (BMC) in the 1950s many years ago, I understood speedos had to be acurate within +/- 10 per cent at 30 mph.
Our experience when calibrating instruments for road test was that they were generally accurate at 30 mph and tended to read high at higher speeds.
However, I doubt if the ordinary motorist realises the factors affecting accuracy of speedometers. The ordinary speedometer doesn't sense the linear movement of the car over the ground but senses the rotational speed of the driving wheel.
So the accuracy of the translated reading depends on the rolling radius of the tyre. This varies with tyre pressure, tyre wear and wheel speed - the rolling radius grows as the speed increases.
And, of course, one has to allow for tolerances within the instrument system itself. We are doing well, if in general use, our speedometer is accurate to 10 per cent.
Unless our cars use a laser or radar system to sense road speed we can't argue with the Garda check.
Conor Faughnan of the AA has been on to us about the regulation of speedometers. He says that there is indeed an EU directive, which requires all speedometers to be accurate.
The only discrepancy allowed is if the gauge is reading 15 per cent faster, in other words reading 30 mph when you are actually doing 26 mph. No allowance is given for speedometers that read slower than your actual speed.
• Send your queries to Motors Help Desk, The Irish Times, Fleet Street, Dublin 2 - or e-mail them to motorshelp@irish-times.ie