Joining a busy motorway
Discussion
Interesting situation that arises on my regular commute quite often when joining the M40 at J4 (High Wycombe), heading SE towards London. The slip road is two lane and at the point where it reaches the main carriageway lane 2 of the slip road merges with the motorway while lane 1 of the slip road becomes the new lane 1 of the motorway, thus increasing the width of the motorway carriageway at that point from 2 lanes to 3. As the slip road reaches the main carriageway the dotted line between the two lanes is discontinued, leaving one wide lane which then narrows down to become the regular lane 1 of the motorway.
What happens is that when it’s busy both lanes of the slip road are occupied with traffic, typically accelerating to around 55/60mph but if the motorway is busy with no space for joining traffic to merge you get traffic in lane 2 of the slip road with nowhere to go. What happens then is that vehicles push back into lane 1 of the slip road, just at the point where the lane 1 traffic is accelerating into what is now lane 1 of the motorway itself. At this point the dotted line defining the two lanes has finished, so that all the slip road traffic that has not yet merged onto the main carriageway is effectively occupying a single wide, but narrowing, lane. On the whole drivers are aware of what is happening and manage this situation effectively with cars merging as necessary. From time to time, however, you see some rapid readjustments of speed and some drivers getting upset at what they see as aggressive pushing in.
My approach is, for preference, to keep to lane 1 on the slip because that gives me a clear route ahead. At the same time I try to avoid overlapping with vehicles in lane 2 on the slip to leave room if they decide/need to join me in lane 1. However, as happened this morning, my plan can go wrong when the van in lane 2 ahead of me realises he can’t merge into the motorway traffic and then brakes sharply before drifting left and moving into what was effectively my lane. I don’t want to brake sharply in my turn because of the stream of following traffic so I find myself going up his inside with a narrowing gap! In this morning’s case I accelerated hard to get clear, effectively undertaking him, and got a horn toot for my efforts!
I guess the answer is not only to avoid overlaps but to leave a more substantial gap to the vehicle ahead in lane 2 to allow for that vehicle braking before moving left. As ever, it’s a matter of judgement as to just how big a gap is required to be sensible while also trying to keep with the flow of the rush hour traffic. This is a matter of observation as to what the traffic flow on the motorway is like, what's behind me, and so forth. Generally, it all goes well but this morning's minor incident has left me thinking that I should have allowed for the van braking in my planning. Always something to learn!
What happens is that when it’s busy both lanes of the slip road are occupied with traffic, typically accelerating to around 55/60mph but if the motorway is busy with no space for joining traffic to merge you get traffic in lane 2 of the slip road with nowhere to go. What happens then is that vehicles push back into lane 1 of the slip road, just at the point where the lane 1 traffic is accelerating into what is now lane 1 of the motorway itself. At this point the dotted line defining the two lanes has finished, so that all the slip road traffic that has not yet merged onto the main carriageway is effectively occupying a single wide, but narrowing, lane. On the whole drivers are aware of what is happening and manage this situation effectively with cars merging as necessary. From time to time, however, you see some rapid readjustments of speed and some drivers getting upset at what they see as aggressive pushing in.
My approach is, for preference, to keep to lane 1 on the slip because that gives me a clear route ahead. At the same time I try to avoid overlapping with vehicles in lane 2 on the slip to leave room if they decide/need to join me in lane 1. However, as happened this morning, my plan can go wrong when the van in lane 2 ahead of me realises he can’t merge into the motorway traffic and then brakes sharply before drifting left and moving into what was effectively my lane. I don’t want to brake sharply in my turn because of the stream of following traffic so I find myself going up his inside with a narrowing gap! In this morning’s case I accelerated hard to get clear, effectively undertaking him, and got a horn toot for my efforts!
I guess the answer is not only to avoid overlaps but to leave a more substantial gap to the vehicle ahead in lane 2 to allow for that vehicle braking before moving left. As ever, it’s a matter of judgement as to just how big a gap is required to be sensible while also trying to keep with the flow of the rush hour traffic. This is a matter of observation as to what the traffic flow on the motorway is like, what's behind me, and so forth. Generally, it all goes well but this morning's minor incident has left me thinking that I should have allowed for the van braking in my planning. Always something to learn!
Yes, that's the junction.
I think your analysis is spot on, in that many drivers who take lane 2 of the slip road expect to merge straight onto the motorway without needing to merge with the traffic in lane 1 of the slip. Thus they only move back into lane 1 at the last moment if and when they find there are no available gaps in the motorway traffic. That's the case, notwithstanding that there is the hatched area at the point where the slip meets the main carriageway and which drivers shouldn't be crossing. Partly, that's because at 8.00am in the morning one's concentration at that point on the road is very much on the traffic and on judging relative speeds, distances and intentions, more than the road markings. It does help to explain, however, why it can all get a bit panicky at that point.
Many two lane slip roads make it very clear that traffic has to merge before reaching the major road with lanes merging and keep left signs, together with hatched areas to narrow the lanes down from two to one, all well before reaching the main road. I wonder why they've not done that here - is it because it is, as you say, a lengthy slip road giving plenty of opportunity (in theory) for cars to complete overtakes and move back to the left before they reach the motorway merge point? If so, I'm not convinced it's working!
I think your analysis is spot on, in that many drivers who take lane 2 of the slip road expect to merge straight onto the motorway without needing to merge with the traffic in lane 1 of the slip. Thus they only move back into lane 1 at the last moment if and when they find there are no available gaps in the motorway traffic. That's the case, notwithstanding that there is the hatched area at the point where the slip meets the main carriageway and which drivers shouldn't be crossing. Partly, that's because at 8.00am in the morning one's concentration at that point on the road is very much on the traffic and on judging relative speeds, distances and intentions, more than the road markings. It does help to explain, however, why it can all get a bit panicky at that point.
Many two lane slip roads make it very clear that traffic has to merge before reaching the major road with lanes merging and keep left signs, together with hatched areas to narrow the lanes down from two to one, all well before reaching the main road. I wonder why they've not done that here - is it because it is, as you say, a lengthy slip road giving plenty of opportunity (in theory) for cars to complete overtakes and move back to the left before they reach the motorway merge point? If so, I'm not convinced it's working!
R_U_LOCAL said:
This junction?
https://maps.google.co.uk/maps?hl=en&ll=51.613...
I think there may be a common misconception with these type of junctions which is leading to the problems you have described. Look at the sign to the left of the carriageway. It indicates that the slip road becomes lane 1 and that there is therefore no need for joining traffic to merge with traffic already on the motorway. This type of junction is called a "permanent lane gain junction".
What the sign also indicates is that the slip road has reduced to only one lane at the point where it joins the main motorway. The correct way to join the motorway at this point is for vehicles in both slip lanes to merge into one lane, and then join the motorway in the newly formed lane 1. Any moves to lanes 2 and 3 should come after the join.
The slip road is long, and two lanes allow vehicles to overtake slower vehicles in lane 1 of the slip road, but drivers should treat any overtakes as a completely seperate action from their motorway join and the plan should be to merge into lane 1 at or around the end of the hazard lines.
I don't think you're doing anything wrong in the situation you described, but as is usual these days, you need to think for the other drivers and if it looks like their badly planned joins won't work out, you need to plan for their potential actions which will inevitably involve braking and moving to the left.
I drove this route again this morning and took particular note of the signage and road markings. Basically, there is just the one sign - the one you refer to and which is visible in the Google maps link, albeit it is currently half obscured by a bush. In any event, this sign would be invisible to anyone in lane 2 who is overtaking a truck or large van. Otherwise there is nothing else at all to tell a driver that the two lanes are reducing to one, no signs and no road markings to say that the end of lane 2 is approaching. Thus it is very natural for drivers in lane 2 of the slip to arrive at the motorway joining point in a situation where they are still alongside traffic in lane 1 of the slip and therefore needing to merge into lane 1 of the motorway with some urgency! Either that or seek to move back left to stay in the lane that becomes the new lane 1 of the motorway. As ever, regular drivers know the score but for anyone new to it (and of course anyone who knows it but doesn't mind being aggressive) in busy traffic conditions it can easily all get a bit tense!https://maps.google.co.uk/maps?hl=en&ll=51.613...
I think there may be a common misconception with these type of junctions which is leading to the problems you have described. Look at the sign to the left of the carriageway. It indicates that the slip road becomes lane 1 and that there is therefore no need for joining traffic to merge with traffic already on the motorway. This type of junction is called a "permanent lane gain junction".
What the sign also indicates is that the slip road has reduced to only one lane at the point where it joins the main motorway. The correct way to join the motorway at this point is for vehicles in both slip lanes to merge into one lane, and then join the motorway in the newly formed lane 1. Any moves to lanes 2 and 3 should come after the join.
The slip road is long, and two lanes allow vehicles to overtake slower vehicles in lane 1 of the slip road, but drivers should treat any overtakes as a completely seperate action from their motorway join and the plan should be to merge into lane 1 at or around the end of the hazard lines.
I don't think you're doing anything wrong in the situation you described, but as is usual these days, you need to think for the other drivers and if it looks like their badly planned joins won't work out, you need to plan for their potential actions which will inevitably involve braking and moving to the left.
I take your point about this being in the nature of slip roads but I think what makes this case different is Reg's observation that the theory is that all traffic on the slip moves to lane 1 (of the slip) before reaching the motorway carriageway. In practice there is almost nothing to say that this is what's intended and a great many drivers therefore stay in lane 2, expecting to merge onto the motorway in the usual manner. When they find there are no gaps in the motorway traffic and there is a very limited distance in which to manoeuvre when compared to most slip roads, they then squeeze left into the "permanent gain lane", causing drivers already there to take avoiding action. Where there is no "permanent gain lane", i.e. on the majority of motorway slip roads, then everybody knows they have to merge onto the carriageway and plan accordingly, whether there are two lanes on the slip or one.
OK, what I think is happening is that on a "standard" 2 lane slip road joining a motorway with no permanent lane gain, if the motorway is busy everyone on the slip - in both lanes - expects to have to slow, possibly right down to a crawl or dead halt if that's what's required to match the speed of the motorway traffic and to be able to merge. As a result, everyone more or less moves together.
In the case of J4 on the M40, however, then those vehicles that have stayed out in lane 2 of the slip, if they discover that the motorway is full such they can't easily merge, realise that if they keep left they can carry straight on using the new permanent lane. Space for them to identify this situation is short, however, so they end up first braking and then rapidly seeking a gap to their inside, which is not an area they've been focusing on up to that point. Generally it all works out with a bit of give and take but no wonder you get the odd moment of panic in evidence. Advanced driving, it's not!
In the case of J4 on the M40, however, then those vehicles that have stayed out in lane 2 of the slip, if they discover that the motorway is full such they can't easily merge, realise that if they keep left they can carry straight on using the new permanent lane. Space for them to identify this situation is short, however, so they end up first braking and then rapidly seeking a gap to their inside, which is not an area they've been focusing on up to that point. Generally it all works out with a bit of give and take but no wonder you get the odd moment of panic in evidence. Advanced driving, it's not!
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