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Comment by marktolson | Hacker News Reader

My friend was using adaptive cruise control while towing a caravan. A car merged in front of him, his car applied heavy braking to maintain the desired distance (not necessarily to avoid a crash) but didn't compensate for the weight of the caravan. Almost lost traction, lead to heavy swerving. He's vowed to never use it again.

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The use of ACC while towing is explicitly discouraged by my car's manual. There are also a number of other considerations that should be taken into account. These systems are practical, but not magic and users should try to understand them well before using.

From the description it sounds like your friend was driving a car that could not safely perform emergency braking. Is that even road-legal anywhere?

To be fair most people who are driving cars aren't towing caravans, so this seems like an extreme edge case.

I heavily use adaptive cruise control and I've never had any issues with it, though I'm always ready to take over in case anything complicated starts happening. It's a nice feature because it saves a lot of energy when the driving is very easy, and then you just drive normally otherwise.

> this seems like an extreme edge case

Most ships don't sink, but we still have lifeboats. Most jet engines don't fail, but we still train pilots to land without them.

Its edgecases that kill.

In this particular edge-case, it's the user fault for using a technology not supposed to be used when towing.

Manufacturer could include a sensor in the towbar.

Did the manufacturer think about this and include a warning? If not, how should the user know?

> If not, how should the user know?

The standard move is to put this in the user manual.

Especially for something as complex as towing, reading the manual should be mandatory.

I think the new F-150 lightning (and I would think equivalent ICE models) can handle that scenario.

I would think the manual likely said not to use it while towing. I’m not sure why one would be afraid of it when used correctly because it didn’t handle something it wasn’t designed to well.

Doesn't his car have ABS? Even under heavy braking, the car's wheels shouldn't lock up and lead to swerving. Did the trailer have its own brakes?

As everyone is pointing out to you, ABS solves the problem of your car's wheels skidding, leading to loss of control.

It doesn't help with the trailer that you're towing failing to stop as fast as you do, trying to run into you, and trying to turn to the side when it can't.

The solution to the latter problem is to be aware that you have the trailer, and to not brake faster than it can handle. The fact that the trailer makes it harder for you to avoid an accident is exactly why many jurisdictions put extra restrictions on cars towing trailers (for example in CA they aren't supposed to go over 55 mph - even on the freeway).

If ACC speed adjustment let to the trailer losing traction and swerving behind him, then there's something not right with his towing setup, even in a panic stop, I've never had my trailer swerve behind me. That's a serious hazard, ACC speed adjustment braking should not cause any loss of control.

I've seen pickups with too little weight on the rear axle lock up the wheels and get pushed around by the trailer especially when the trailer braking is adjusted too light, but with the advent of ABS, that's not nearly as common.

The problem here isn't the ACC braking (even though many car markers will tell you not to use it while towing), it's trailer setup.

At least in the UK, you're not supposed to have a trailer of such a weight that it would destabilise you if you emergency brake. That's dangerous and should be outside of the vehicle's/driver's specifications. Above a certain weight, you should be using a braked trailer.

Does ABS help if the car is braking faster than the trailer?

I believe you are thinking of skidding, not swerving.

I don't see how the trailer can force a car to swerve without any loss of traction through skidding.

Trailers normally have back-up brakes that engage as soon as there is pressure rather than tension on the tow bar. To allow reversing they also have a block that allows you to disable the backup brake (usually a lever that moves in to avoid the compression of the tow bar). I wonder if your friend had accidentally forgotten to re-enable the automatic brake after getting ready for the trip.

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