Polarised filters on traffic lights in Dublin

AndrewJRenko

Does anyone know the purpose or function of the polarised filters that seem to be getting fitted on some traffic lights around Dublin.

I've seen these at the junction at AIB Donnybrook, and at Rathmines bridge. For cyclists coming over the bridge and heading south, it makes the green light almost invisible, as the filter seems to be directing the light back over the cyclists head towards the centre of the traffic lane, not usually where a waiting cyclist will be.

How are these supposed to work?

ted1

AndrewJRenko wrote: »

Does anyone know the purpose or function of the polarised filters that seem to be getting fitted on some traffic lights around Dublin.

I've seen these at the junction at AIB Donnybrook, and at Rathmines bridge. For cyclists coming over the bridge and heading south, it makes the green light almost invisible, as the filter seems to be directing the light back over the cyclists head towards the centre of the traffic lane, not usually where a waiting cyclist will be.

How are these supposed to work?

Stops cars speeding up to catch the green light

AndrewJRenko

ted1 wrote: »

Stops cars speeding up to catch the green light

How?

ted1

AndrewJRenko wrote: »

How?

If they are approaching a green light and see it form a distance they will accelerate to try and get through before it switches

AndrewJRenko

ted1 wrote: »

If they are approaching a green light and see it form a distance they will accelerate to try and get through before it switches

I'm not sure I get it - the green light is projected back away from the junction, so if anything, it would facilitate these drivers rather than hinder them.

AndrewJRenko

Have a look at these pics from Rathmines Bridge, showing the difference in the red (unpolarised) and the green (polarised). In the 'green' pic, you can clearly see the green light on the opposite corner of the junction, but the polarised green light looking back towards the bridge is not visible at all to anyone one the bridge, close the the kerb. I swear it is actually green, you can see bits of green as you get closer or move out a bit from the kerb.

What's going on here?

GM228

They are known as Optically Programmed traffic lights/signals.

Something I read from a member of the Institute of Transportation Engineers which will answer your question.

These typically consist of signal heads with two polarized lenses - rotate these lenses and by how they are aligned you can change where the signal is seen from.

The typical application for these programmed heads is in locations where there is a concern that the signal head can be seen from an adjacent lane group or closely spaced traffic signal. The concern is that a driver will look up, see green on the adjacent signal or lane group and continue through a red light.

For instance, if you have two closely spaced intersections, you may want to keep the driver from seeing the downstream set of signal indications until after they pass the upstream set. This prevents the case where the driver looks up, sees the green indication on the downstream intersection and runs the red light at the upstream intersection.

MOH

GM228 wrote: »

They are known as Optically Programmed traffic lights/signals.

Something I read from a member of the Institute of Transportation Engineers which will answer your question.

Ah, that would actually make sense on the far side of the bridge, heading inbound - I can easily see how a driver approaching the canal inbound could follow the green at the junction on the far side of the bridge even if there was a red at the canal junctions.

But in the OPs pics I can't see the green at all. Not sure what the point of a polarising filter there is.

ED E

Its pretty obvious lads if you step back a little.



Coming up to the bridge on the nearer stop line theres no left side light, but only ~8m forward theres the green Andrews asking about. If a driver doesnt look sharply right(Google cam is much better positioned than a driver) then it would be easy to assume that green was yours potentially sending you through:
A. Canal traffic
B. Cycle corridor traffic
C. Ped crossing

The way the cycle is set currently that possibility should never arise as they're chained but in future if it was modified having the filter there makes sense.

The fix is a pole mounted cycle light.

AndrewJRenko

GM228 wrote: »

They are known as Optically Programmed traffic lights/signals.

Something I read from a member of the Institute of Transportation Engineers which will answer your question.

Thanks, that's very helpful.

ED E wrote: »

Its pretty obvious lads if you step back a little.



Coming up to the bridge on the nearer stop line theres no left side light, but only ~8m forward theres the green Andrews asking about. If a driver doesnt look sharply right(Google cam is much better positioned than a driver) then it would be easy to assume that green was yours potentially sending you through:
A. Canal traffic
B. Cycle corridor traffic
C. Ped crossing

The way the cycle is set currently that possibility should never arise as they're chained but in future if it was modified having the filter there makes sense.

The fix is a pole mounted cycle light.

It looks they just haven't thought about cyclists at all, at a junction adjacent to the busiest dedicated cycle track in the city. The marked location for cyclists is in the inner lane. And yet, the lights are designed that cyclists will not be able to see their green light from that location.

Unless the markings have changed since the last time the Google camera car visited, both lanes are marked with ground arrows as being suitable for traffic heading straight on.

https://goo.gl/maps/r1Hpxtw1cGw

And yet, traffic on the inside lane won't see the green light for straight on.

I'm struggling to see why this arrange is better than two simple filter arrows - one pointing straight ahead and one pointing left.

ED E

Most of DCCs layouts have cycling as an afterthought. Lots of stupid examples. Same goes for FCC/DLRCC/SDCC.