OK, if we assume a 'survival of the fittest' rationale, what happens if you and your significant other are in different locations when things start to unravel ?
How do you coordinate your movements ?
What happens if you have young kids ?
What happens if you have an infirm parent/grandparent living with you ?
A slow moving but high load bearing car suddenly becomes a lot more realistic and appealing.
Personally I think that all modes of transport will cause equal chaos - roads will be clogged with cars and larger vehicles, tracks and trails could be clogged with motorbikes, horseback riders, mountain bikes and people on foot; basically the world and his wife will all be trying to get some place, all at the same time.
A hard walk with limited supplies will cover nominal mileage per day. A lot of gear/kit/supplies can be carried with multiple people over long distances with little effort, even if traffic is moving slowly.
Absolute gridlock is something else entirely though, at which point some pretty horrific decisions would have to be made, but for anyone intending to get out of Dodge with their family unit intact this will normally involve a vehicle, multiple people and a lot of stuff.
Keep in mind that a vehicle can be stripped for bits, it offers rudimentary shelter from the elements, warmth, light etc, and if yours is reliable and not a gas guzzler you can cover a lot of ground on a single tank of fuel, especially in relation to a small island like Great Britain or an even smaller one like Ireland. It's tough to consider how you could easily replicate that combined shelter/load carrying capacity and range with what you can hump on your back.
Me ?
A fast exit over distance is impossible due to my 3 year old daughter and leaving her behind is no option at all. I can't easily carry a heavy or even reasonably well loaded pack AND her over significant distances or difficult terrain, so, what are the realistic options ?
This is what I was getting at earlier.
Testing your response time for exiting your home is a great exercise, especially for fire readiness. I am comfortable that I can get myself and my daughter out of the house very, very quickly. After that your movements depend largely on the nature of the threat and where you hope/expect to be able to weather the storm (no pun intended).
Once you've tested your exit strategy for getting out of the house or place of work quickly about all you can do is a timed yomp to your planned destination which, under emergency circumstances (especially those that are weather related) will almost always be significantly slower due to adverse conditions.
Exiting your home is the first and (arguably) most important step. After that, everything depends on the nature of the threat and whether everyone else is having the same "let's get out of here" idea.
I keep a bug out bag in the boot of the car and another in the house.
I also have a few small caches of supplies in the garden adjacent to the house and in similar places at my cottage on the Scottish Borders. Even if I can't get into those properties I can at least replenish supplies from their surrounds.
Beyond that I am relying on being able to scavenge most of what I need a week or two after Sainsbury's runs out of food which, these days, is about the greatest threat faced by folks in the British Isles
One factor often NOT discussed instead of bugging out is the whole idea of whether to stay and, if you do, how best to go about it. Bugging out would necessitate having some threat or other that absolutely cannot be handled by staying. That's a tough one to visualise in the UK and surrounding islands. Keep in mind that bugging out leaves behind the very best shelter and resource you have - AKA your home.
It's interesting to think about the various possibilities that could cause any kind of exodus, and how we would cope with things in the event of major change.
I do feel that far too much emphasis is placed on leaving everything behind and going somewhere else in some kind of semi-self-sufficient style (all somehow contained in a rucksack) when, in fact, staying put could well be the best option in many emergencies and locations. I think there is as much if not more mileage in preparedness at home as there is in gearing up for bugging out and here in the UK, other than for domestic fires or, in some areas, perhaps floods, there aren't too many circumstances one could imagine having to scoot from, which I am thankful for
