Tigertails without lane drop

[ColinB]

By which I mean motorway exits with two diverges from lane 1 to the same destination e.g. M26 exit from M20 (TD22/06 layout B1). Why do we have them, are they really useful? Somebody must think so because they are proposed within both the M25 J10 and M42 J6 projects.

I know TD22/06 recommend them in some situations where the diverge flow is between 1350 and 1800/h (Fig. 2/5MW). However, this is only because it assumes a one lane slip road has a lower capacity than a lane on the mainline or a two lane slip (1350/h cf. 1800/h), so a second exit route is needed. But this cannot be true. If 1800/h can travel along in lane 1 there is nothing to stop all 1800 diverging onto the slip road. I can see that at a busy exit (i.e. diverge flow greater than 1800/h or less if some through traffic remains in lane 1) the second diverge could allow traffic from lane 2 to swoop across. But clearly this is not the design intent. If you wanted traffic from lane 2 to exit you would have a lane drop and diverge (layout D1).

So can anyone explain why you would ever want a B1 layout?

[Bomag]

It's to maintain diverge capacity. If you have a single diverge leading to a two lane slip road then drivers who diverge at the start of the taper and go into lane 2 of the slip road and drivers who exit just befor the nose heading for lane 1 of the slip road reduce capacity. If there is no need for signifcant weaving on the slip road then it is less likely to cause conflict in the diverge taper.

[the cheesecake man]

...

[ravenbluemoon]

A lot of the junctions on the widened M1 through Notts/Derbyshire feature these kind of tiger tails. Certainly J26 and 27 have them.

[Bryn666]

How much use does the second exit actually get though given the acres of hatching will put the hard of thinking off?

M6 J34 has this layout now and most people seem to leave at the first exit lane and ignore the second completely.

[SouthWest Philip]

In the absence of a lane drop for the first exit, surely this kind of layout just encourages poor driving and cutting in from the middle lane at the last moment to take the second exit?

[Johnathan404]

I'm willing to believe it does increase capacity, but by a very small margin. 'Blood out of a stone' is the DfT roads mantra, after all.

With a blank canvas and a blank cheque, a very wide deceleration lane like M56/M6 would be better, surely?

[Ruperts Trooper]

In the absence of a lane drop for the first exit, surely this kind of layout just encourages poor driving and cutting in from the middle lane at the last moment to take the second exit?

If the exit slip has two lanes, then mainline traffic from both lanes 1 & 2 need to be able to access it - mainline lane 1 takes the 1st exit lane giving mainline lane 2 chance to move over into lane 1 and then take the 2nd exit lane.

[ravenbluemoon]

In the absence of a lane drop for the first exit, surely this kind of layout just encourages poor driving and cutting in from the middle lane at the last moment to take the second exit?

Admittedly I used to do this to avoid the pootlers using the first lane exiting J26, but I was turning right at the junction anyway. You had to be aware of those who'd drift across to the right when the two lanes met however.

[Peter Freeman]

In the absence of a lane drop for the first exit, surely this kind of layout just encourages poor driving and cutting in from the middle lane at the last moment to take the second exit?

If the exit slip has two lanes, then mainline traffic from both lanes 1 & 2 need to be able to access it - mainline lane 1 takes the 1st exit lane giving mainline lane 2 chance to move over into lane 1 and then take the 2nd exit lane.

I don't think that's the designer's intention. Surely that would only be the intention if the first diverge was a lane drop, leaving lane 2 as the new mainline left lane. I'd call the practise poor driving, as suggested by Southwest Phillip, because you may be in conflict with a stream of lane 1 traffic that has not exited but is continuing ahead. However, this might not be too bad if there is sufficient distance between diverge 1 and diverge 2.

In the absence of a lane drop for the first exit, surely this kind of layout just encourages poor driving and cutting in from the middle lane at the last moment to take the second exit?

Admittedly I used to do this to avoid the pootlers using the first lane exiting J26, but I was turning right at the junction anyway. You had to be aware of those who'd drift across to the right when the two lanes met however.

I think the 'pootlers' still have a right to safely move ("drift" …?) to the right lane of the off-ramp, even though they have taken the first diverge. And how many seconds have you shaved off your trip time?

[Ruperts Trooper]

In the absence of a lane drop for the first exit, surely this kind of layout just encourages poor driving and cutting in from the middle lane at the last moment to take the second exit?

If the exit slip has two lanes, then mainline traffic from both lanes 1 & 2 need to be able to access it - mainline lane 1 takes the 1st exit lane giving mainline lane 2 chance to move over into lane 1 and then take the 2nd exit lane.

I don't think that's the designer's intention. Surely that would only be the intention if the first diverge was a lane drop, leaving lane 2 as the new mainline left lane. I'd call the practise poor driving, as suggested by Southwest Phillip, because you may be in conflict with a stream of lane 1 traffic that has not exited but is continuing ahead. However, this might not be too bad if there is sufficient distance between diverge 1 and diverge 2.

It works well enough for me - the mainline traffic in lane 1 thins out enough and the first exit slip to make it an easy, safe manouevre from lane 2 > lane 1 > 2nd exit slip - especially if the queue backs up in the mainline lane 1.

No point having 2 exit lanes if everyone has to use one slip lane.

[ravenbluemoon]

In the absence of a lane drop for the first exit, surely this kind of layout just encourages poor driving and cutting in from the middle lane at the last moment to take the second exit?

Admittedly I used to do this to avoid the pootlers using the first lane exiting J26, but I was turning right at the junction anyway. You had to be aware of those who'd drift across to the right when the two lanes met however.

I think the 'pootlers' still have a right to safely move ("drift" …?) to the right lane of the off-ramp, even though they have taken the first diverge. And how many seconds have you shaved off your trip time?

Drift: Moving from the first lane to the second without using mirror or signals. I don't dispute them not having a "right" to change lane, but those looking beyond their bonnet can see what the road is doing, and can plan accordingly for the least conflict.

In peak hours choosing the correct lane to turn right could save in the order of minutes due to the volume turning left in a single lane. Or saves a risky manoeuvre trying to change lanes from a static queue into moving traffic.

[Peter Freeman]

But couldn't you gain most of that queue-avoiding advantage by staying in lane 1, passing by the first diverge, and entering the second diverge for your lane-2 queue on the off-ramp? Yes, you'd still crawl for a while if the queue reached onto the mainline, but after that you'd pass a large proportion of the 'pootlers' simply by being on the right side of the tiger tail.

Not having actually seen the situation there, I may well be misjudging it.

[ravenbluemoon]

It could be done if not too busy, and a reasonable judgement could be made while driving in lane one of the motorway as to how much was turning off. Given the lane discipline of most D4 motorways, the majority of the non-HGV will be turning off, so once they have, there's often a nice free run to the second diverge. I'm not of the type to swerve across from lane 2 after the 100m countdown marker

Much prefer that method to having to change lanes from a standing start later. It's not like I'm unsafely blasting along the slip road, but others do.

[Stevie D]

I think the 'pootlers' still have a right to safely move ("drift" …?) to the right lane of the off-ramp, even though they have taken the first diverge. And how many seconds have you shaved off your trip time?

The reason for having two exit choices is to increase the capacity of the exit, which means they must be anticipating some drivers will take the second exit, and probably from lane 2 (or else the capacity of the exit is still limited to the capacity of a single lane minus all the drivers in lane 1 and going straight on.

The sensible thing for drivers to do, if they know they need to be in the right-hand lane on the slip-road, is to take the second exit so that they are in the correct lane straight off, and then they won't need to change lanes on the slip-road. Some junctions indicate this on the signs from the mainline, but not all do.