Two Phase Traffic Lights

The study of British and Irish roads - their construction, numbering, history, mapping, past and future official roads proposals and general roads musings.

There is a separate forum for Street Furniture (traffic lights, street lights, road signs etc).

Registered users get access to other forums including discussions about other forms of transport, driving, fantasy roads and wishlists, and roads quizzes.

Moderator: Site Management Team

Post Reply
tommyd49
Member
Posts: 835
Joined: Thu May 06, 2004 16:09

Two Phase Traffic Lights

Post by tommyd49 »

On my local primary route, the A10, two sets of traffic lights have been put in recently, at Butt Lane and Denny End Junction. In my mind, they're a bad idea, but that's me :D . Anyway, they are both 3 way junctions, with the A10 the main road going straight through and minor roads into Histon for the Butt Lane junction (although most traffic to Histon uses the B1049 off the A14) and Waterbeach for the Denny End junction. Instead of having two phase lights (one phase for traffic on the main road, the other phase for traffic on the minor road), they are 3 phase.

Now, coming northbound on the A10, the minor road turn off at Denny End Junction is on the right. I'll use this example rather than Butt Lane. Phase 1 is for traffic on the main road to go straight or turn left - no turning right onto the minor road. The second phase is for traffic on the northbound side only - traffic on the southbound side is stopped. The third phase is for traffic on the minor road to turn either left or right. The first and second phase bug me - having this system means that traffic backs up on the A10 southbound backs up at busy times. I think if traffic lights are absolutely essential here, they should be two phase to let A10 southbound traffic flow - and there is a lot of it in the morning heading down to Cambridge.

My question is - is there some kind of discouragement of two phase signal these days?
User avatar
PeterA5145
Member
Posts: 25347
Joined: Sun Mar 24, 2002 00:19
Location: Stockport, Cheshire
Contact:

Post by PeterA5145 »

Surely the purpose of having three-phase lights at T-junctions is to provide a specific phase for vehicles turning right from the main road to the side road. Otherwise they would have to wait for a gap in the oncoming traffic and a tailback could develop.

This kind of arrangement is very common and where there is a substantial right-turning flow entirely sensible.
“The human race divides politically into those who want people to be controlled and those who have no such desire.” – Robert A. Heinlein
User avatar
kieron
Member
Posts: 1102
Joined: Tue Jun 14, 2005 00:29
Location: Connah's Quay

Re: Two Phase Traffic Lights

Post by kieron »

tommyd49 wrote:My question is - is there some kind of discouragement of two phase signal these days?
Three phase lights ban turning traffic from being in the junction whenever conflicting traffic is allowed to enter it, so remove one potential cause of accidents.

If the main road is too narrow for turning lanes, waiting turning traffic can also block through traffic, so that the second phase needs to be much longer than turning traffic needs by itself. The three-phase lights at the crossroads in Northop are like this, giving northbound traffic a phase to itself.

I don't know how wide the A10 is by here, however.
User avatar
sotonsteve
Member
Posts: 6079
Joined: Mon Aug 08, 2005 21:01

Post by sotonsteve »

A crossroads near my house was converted from three phase to four phase. The phases were originally along the main road in both directions, allowing vehicles to turn left and right at the same time. The other two phases were for the two side turnings. With the modification cars on the main road cannot turn right without oncoming traffic being stopped at red lights and a right turn green arrow lighting up.

The junction used to flow fine, but now the phasing has been altered the capacity of the junction is lower and congestion is more common. I wished they could have applied the logic of "if it ain't broke don't fix it". Policy nowadays is "let's make life harder and squander taxpayers' money at the same time".
tommyd49
Member
Posts: 835
Joined: Thu May 06, 2004 16:09

Post by tommyd49 »

PeterA5145 wrote:Surely the purpose of having three-phase lights at T-junctions is to provide a specific phase for vehicles turning right from the main road to the side road. Otherwise they would have to wait for a gap in the oncoming traffic and a tailback could develop.

This kind of arrangement is very common and where there is a substantial right-turning flow entirely sensible.
The problem is while it gives the traffic turning right from main road to side road a certain chance to get out, it causes big problems for traffic heading south on the A10, having to stop for a long period of time. Whilst at the Denny End junction (not at Butt Lne though), there is a fair amount of traffic turning on and off Denny End Road, the fact is the lion's share of traffic on the A10 heads straight. So the 3 phasers cause bigger tailbacks than if the junction had 2 phasers (or no lights at all :D).
boing_uk
Account deactivated at user request
Posts: 5366
Joined: Wed Dec 04, 2002 16:01

Re: Two Phase Traffic Lights

Post by boing_uk »

tommyd49 wrote:My question is - is there some kind of discouragement of two phase signal these days?
No, just plenty of idiots trying to play at being traffic signals engineers.

After all, theyre only bulbs at the end of a bit of wire arent they...
Post Reply