Self-driving cars aren’t science fiction – they’re already here. I’m not just talking about Google’s autonomous car undergoing evaluation on public roads; shop at a Mercedes-Benz or Tesla showroom and you can already buy cars which drive themselves, albeit for short periods. But as yet, they won’t drive themselves while towing.

I’ve just spent a day driving the new Mercedes-Benz E-Class. It has the makings of a very impressive tow car – I’ll come on to that later – but its stand-out feature is semi-autonomous driving.

Spend £1695 on the Driving Assistance Plus package, and the car comes with several driver aids including Drive Pilot. This uses adaptive cruise control to monitor the distance to the car in front, and employs a stereo camera and radar sensors to pick up road markings. It goes further than systems which warn the driver if they are drifting out of their lane, and will actually steer for them, keeping the car in the centre of the lane with no input from the driver.

So, the system takes care of both the speed and direction of the car. It will do so with the driver’s hands off the wheel, although after a few seconds the E-Class displays a warning on the dash asking the driver to put their hands back on the wheel. If the driver ignores this warning the car will eventually slow to a halt with the hazard lights on.

The handbook is quite clear about the safe way to use the system: the driver should keep their hands on the wheel. It’s important to do so, as Drive Pilot has its limitations. A little green steering wheel on the dashboard shows when the system is active, but if it loses track of the lane markings it turns to grey. That’s all the warning you get that Drive Pilot is no longer steering the car, so there’s no reading the business section in the outside lane if you want to reach your destination in one piece.

Drive Pilot is also limited in the severity of the bends it can cope with. So, while it will happily drive on multi-lane A-roads and motorways with gentle curves, it will drift wide on tighter bends, even if the lane markings are clearly in view.

So, if the driver doesn’t use the system properly or overestimates its abilities, there are potential dangers. It’s fine if you follow the advice in the handbook and keep your eyes on the road and your hands on the wheel, ready to take over steering at a moment’s notice. But then, what’s the point? If you are concentrating just as if you were driving yourself, how is Drive Pilot of any benefit? It’s an impressive piece of technology, but if it’s supposed to make driving more relaxing, then it fails.

I found it disconcerting to be on constant alert, waiting for the little green light to turn grey, while simultaneously trusting my electronic co-pilot to make safe decisions. And however accurately the system steers, it never quite puts the car on exactly the same bit of tarmac the driver would have chosen. There’s always a doubt as to whether that’s because the system knows best, or if it’s about to hand full control back to the driver.

Another drawback (or blessing, depending on your point of view), is that Drive Pilot won’t operate when the E-Class is towing a trailer. While it may not be top of the list of challenges for autonomous driving engineers to overcome, it’s something car companies will have to face when fully autonomous cars go on sale. How will an autonomous car deal with the very specific demands of towing, and cope with any instability?

For now, even if I was tempted to spend the £1695 to enjoy the other safety systems which are bundled in with Drive Pilot, I can’t imagine wanting to use it.

I wouldn’t usually spend so long describing an optional extra, but I find the technology intriguing and frustrating in equal measure. But now, let’s forget about Drive Pilot for a while and concentrate on the car.

The E-Class is very good indeed. In fact, it’s a shame Mercedes-Benz didn’t have a car available in time for the Tow Car Awards 2016 testing, as although I still haven’t had an opportunity to tow with one, I don’t doubt it would have been a contender.

For now there are two engine options: the 191bhp E220d and the 254bhp E350d. I drove the less powerful version, but with 295lb ft of torque it’s quick enough for most tastes, and the nine-speed automatic gearbox makes the most of the engine’s muscle.

It should easily be powerful enough to pull any suitably matched caravan. The kerbweight is a healthy 1680kg, which gives an 85% match figure of 1428kg. The legal towing limit is 2100kg.

Official economy and emissions vary depending on the size of the alloy wheels fitted, but 72.4mpg on the combined cycle and carbon dioxide emissions of 102g/km are possible with 17-inch alloys. These are remarkable figures for a car of this size and performance, even allowing for the expected drop-off in real-world economy; I saw better than 50mpg on my test drive.

The new Mercedes E-Class drives very well. It may not be as sporty as the Jaguar XF, and the steering lacks the Jaguar’s precision and feel, but it’s a very composed and comfortable car. That bodes well for stability when towing.

Keen drivers may prefer the Jaguar, but inside the cabin the E-Class shows the XF a thing or two. It’s beautifully made. The two huge 12.3-inch displays look crisp and clear, although it’s worth noting that both are optional extras.

There’s plenty of space front and rear, too, although the panoramic glass roof fitted to the car I drove does steal some rear headroom. The boot is a healthy size, too (540 litres), although if more luggage space is needed the estate version will arrive in showrooms this autumn.

Even compared with prestige rivals like the Audi A6, BMW 5 Series and Jaguar XF, the new E-Class is relatively expensive. Prices start from £35,935, and including options, the car I drove would set you back £46,655.

Even at this price, though, the E-Class is neck-and-neck with the best executive saloons, and beats them for ride comfort and interior quality. But if you do specify Drive Pilot as an option, you’ll enjoy the E-Class more if you drive it yourself.