Lots of motorways have 'tramlines' in the inside lane caused by HGVs. These have a wider track than your average car and can cause the effect you describe. They can be more easily seen on a wet road as the 'troughs' fill with water.

M20 used to be really bad for tramlining and often threw the car steering from one direction to the other in quick succession, most drivers kept the car outside the lines to prevent problems.

I think they have resurfaced some of it now as they had roadworks around Maidstone area which was one of the worst areas for TL

First place i'd look is the tyres, are they unheard of junk, check pressures check tread for uneven wear check directional tyres are facing the right way (directional will have an arrow on sidewall showing forward rotation), are they a matched pair preferably matched four, don't forget to check the rear tyres they can have more effect on steering than many people expect.

If the tyres are all good it sounds to me like classic symptoms of wheel alignment out, depending on friction of surface if you run with one wheel on a painted white line that line will have less grip (especially when wet) than the other running on decent tarmac, hence the car will prefer to go in the direction the tyre with the best grip has, as said above tram lines caused by heavy traffic can have a similar effect.

Also check for worn bushes in the suspension, alignment might be correct but as speed increases so does the friction force tending to push your tyres back which knocks the alignment straight out, if you have a weak bush in say a front wishbone the wheel that side would tend to toe out (more than likely) under the forces...a good chjeck for worn bushes is to apply the park brake and lock the steering, then get hold of each wheel and attempt to roll the car down the road by rotating the said wheel each way, if the wheel moves excessively in relation to the wheel arch you might have worn bushes...BMW 3 series were particularly easy to diagnose for bush wear like this.

Newish car so wear should not be an issue, but maybe someone whacked a kerb or pot hole and something's slightly out of line, only needs to be by a small amount you may not see with he naked eye.

Going on and off the throttle can produce a similar effect, if the car wants to move over when you do this there's something wrong tyre or geometry wise.

Edited by gordonbennet on 18/09/2020 at 08:43

In addition to above good suggestions, is your car fitted with "lane assist" ?

These symptoms are exactly what "lane assist" does. The steering will resist your attempt to change lanes unless you indicate the move first, I think.

Read the manual to confirm this is the set up on your car.

Yet another "improvement" imposed by the nanny state health and safety lobby, for which the customer has no choice to decline it and pay less.

OP says above he was not using lane assist.

Whatever it is.

I was sent on a familiarity course for my current lorry, ongoing test bed for a roll out of a new fleet due soon, the chaps supervising the course told me the next update, due later this year or early next, the new vehicles would be interfering with the steering, well the computers on board would.

44 tonners steering themselves, what could possibly go wrong.

Edited by gordonbennet on 20/09/2020 at 09:01

DS3 Crossback has lane keep technology as standard. Indicate to change lanes and it won't intervene. It can probably have it's sensitivity decreased via a menu, or switched off, but it'll probably come back on every time the car is restarted.

Who on earth would want a car that interferes with the steering, you couldn't pay me enough to have such a pointless thing, auto braking is frightening enough.

nips off to spray some more rustproofer liberally about, stave off the day i might have to accept this electronic tat.

Edited by gordonbennet on 18/09/2020 at 15:48

Just use your indicators when changing lane and then the system knows that you wanted to change lane and will not do this. It is the correct way to drive anyway.