Slip roads onto dual carriageways and motorways are there to be used. They are usually long enough to allow even the most underpowered of vehicles a reasonable chance of reaching 60/70mph before joining the main carriageway and for a motorcyclist they provide a hive of opportunity.
The most common point of discussion in debrief is that the candidate has ridden approximately halfway along the slip road, into the zone separated from the main carriageway by the dotted white line, and then having looked over their shoulder and seen an empty carriageway they make a course change out into lane one. They do this despite there being no vehicles ahead of them on the slip road, or any coming up behind them on the main carriageway. The question I always ask, making reference to the system of motorcycle control (IPSGA), is what information warranted the change of position (or course). If there are no other vehicles to influence the current course why not continue in a straight line and emerge naturally onto the carriageway at the end of the slip road?
This basic principal can then be developed to introduce opportunity. If you are unfortunate enough to follow a car, van, lorry, or even a slower motorcycle onto the slip road there is no advantage to be had by staying close behind it. If you do you will have to worry about what they are doing ahead of you whilst trying to look back down the carriageway. Given the difference in acceleration rates between the average bike and the average car/van/lorry your riding plan at this stage will probably include some sort of overtaking move. If you can't get past immediately, ie: whilst still within the protected portion of the slip road (and you frequently can provided it is wide enough) then hang back and let them sort themselves out. Use this opportunity to get an early view past them of the slip ahead, and also back down the carriageway. If both are clear it is extremely likely that the vehicle ahead of you is going to pull straight out at their earliest opportunity (as per first paragraph). If they do it is highly probable that all you will need to do is continue accelerating and go straight past them, using the slip road for exactly what it was designed for and emerging ahead of them. This move is particularly effective if there is a slower vehicle already in lane one and the vehicle ahead of you pulls out behind it, the driver then continuing out into lane two in order to overtake and therefore blocking any offside route to you. One word of caution though, be extra careful if the vehicle you are overtaking is a large vehicle that could possiby be shielding another vehicle. You don't want to arrive on their front nearside to find no gap, or even another vehicle returning to lane one from the offside.
This may sound all a bit adventurous but if you consider the alternatives it makes sense. Do you really want to be in the position of emerging onto a dual carriageway into lane one directly behind a slow moving vehicle, and then having to make a separate move out into lane two, possibly having to try and adjust speed whilst looking in the mirrors and merging with overtaking traffic. It is far easier to merge with traffic ahead that that coming up behind, but you can only do this if you have allowed space between yourself and the vehicle ahead.
Remember, this tip is not compulsary, and circumstances won't always allow it. Keep it in mind as an option and look for opportunity to use it. It can be very effective..... as some associates have found out when I do it to them!!