Ok, I'll go along with the 45 degree angled jet, say laid along the "C" pillar lines of a typical hatchback car. When you actuate you get (say) 50% reverse thrust and 50% increased downforce, which would work particularly well as it would be at the rear of the car and cause a "squat" effect rather than a nosedive, bringing the rear tyres into play much more than normal.
Clearly this would need to be linked up to an EBA type system so that the newfound additional grip would actually be exploited.
Next problem is the propellant. Can any solid or liquid fuel ignite and produce controllable thrust within the sort of time frames required? I say controllable as I've got visions of the system being activated at low speed and continuing to propel the car backwards once it's stopped, like some sort of demented sorcerer's apprentice.
I've already discounted any form of turbine or jet, as clearly it's little use waiting "a few seconds" for it to spool up! How much energy can a small but ultra-high pressure air tank contain? <thinks: SCUBA tank?????>
Some other issues that spring to mind are what happens if it is inadvertedly deployed in normal driving? What if only one side "fires"? What happens if the driver has a need to go from full emergency braking back to full acceleration, eg if it now looks like there might be a chance of the car emerging from the sideroad actually passing behind?
Anyway, back to the general brainstorming thing, here's a further idea:
In dry conditions vehicle braking is pretty damn good, the much more important issue is driver response time. Really, the time braking performance becomes a major issue is when it is compromised by surface conditions, usually water.
So what can we do to make the wet road behave more like a dry one. At the moment the only thing we do is cut tread into tyres. How about (in ascending order of barminess?)...
1. v-shaped deflector "plough" that is deployed in front of each tyre, to fling standing water out of the way, so tyre gets a wet road to bite on, but not actually standing water.
2. some form of rotating sponge that gets similarly deployed, but actually absorbs the water off the road, extracts it from the sponge and flings it clear of the wheel, so tyre gets a moist road instead of a wet one,
3. Infra-red panel and / or flamethrower in advance of the contact patch, to vaporise anything left by 1 and 2.
With all three we could cross standing water with racing slicks, confident that if we need to stop we can get full dry weather performance.
I do
so like brainstorming!!!