The Rail Accident Investigation Branch (RAIB) have issued urgent safety advice to Edinburgh Trams over the warning horn on the trams which has been found to be below levels in line with current industry guidance. This warning comes following the fatal accident in September 2018 when a pedestrian was hit by a tram between Balgreen and Saughton tramstops.
As with most modern trams the CAF built trams which operate the service on Edinburgh Trams have two forms of audible warning – in most cases the warning bell is used but there is also another warning horn, which tends to be used in more urgent situations and which should be at a louder volume. However, during investigations into the accident back on 11th September 2018 the RAIB discovered that not did the horn not reach a sound pressure in line with current industry guidance but also that it was no greater than the tram bell.
The RAIB conducted acoustic measurements of both bells and horns fitted to the tram involved in the accident and also on one other tram. Tests from 7 metres in a flat, open area over ballast, found that the arithmetic means of the sound pressure levels from nine tests of the warning horns were approximately 86db(A) and 85db(A) – current guidance states that a level to, or greater than, 93db(A) should be achieved in these circumstances. It is apparently generally recognised that a reduction of 10db is approximately equivalent to a halving in loudness as perceived by the human ear.
Other tests were also conducted for both trams from a distance of 2 metres to compare sound pressure levels of the bells and the warning horns – this found that the average sound pressure level of the bell was 95db(A) and the horn was 89db(A).
Where the accident happened – on a footpath crossing the tramway between Balgreen and Saughton – further tests were undertaken at different distances along with typical background noise measured at the crossing at the same time of day as that of the accident. This found that neither the bell nor horn were significantly discernible above the level of background noise to the indicate the approach of the tram at a full service braking distance from the crossing at line speed. It was also found that the horn was in fact quieter than the bell.
The advice to Edinburgh Trams Limited is that they should increase the sound pressure of the warning horn. In the meantime they should also consider measures to mitigate risks at locations where audible warning may be required – in particular whether the current use of the bell or horn as a method of warning to pedestrians using footpath crossing is appropriate.