320,000 cubic metres of concrete. 30,000 tons of steel. 5km of road deck. 2,434 road deck sections. 1,000 men working for four years. Several milion litres of white paint. Tag on a fantastic bit of Plain Common Sense, and you have an beautiful piece of engineering linking two countries across some pretty hostile water with a 50-foot tidal range. You and I know it as the Second Severn Crossing.

The Severn Crossing is very important to me, primarily because it represents the shortest route for me to get to BMW in Wales - where I am travelling to this evening. Last time I went, I went on the bike, but this time I will be going in the car - The Missus and the Wee Yin are going too. We're staying at Dderi Farm, and they are going to love it. The Wee Yin is dead excited at the thought of staying on a working sheep farm. The Missus is looking forward to a bit of piece and quiet.

As bridges go, there are bigger and there are better. There are bridges across more hostile stretches of water - the Golden Gate being the best example - and bridges that dwarf the vital statistics of the Severn Crossing (think Millau).

What makes the Severn Crossing unique, and this is where the common sense comes into play, is its approach to motorcycles. Today, in the car, crossing the bridge will cross me a fiver. Very reasonable too. Last time, on the bike, I was waved through the toll for free.

Blogs passim tells you the fun and games on tolls if you a re a biker, and Severn River Crossing plc have obviously given this some thought. This is not only better for the car drivers, not getting stuck behind a bike, it's also better for the bridge. Let me explain.

The Severn toll booths are right at the end of the Welsh viaduct, on the westbound carriageway. This means that you cross the bridge, then pay the toll. So if there is a big traffic queue waiting for the tolls, then that traffic queue tails back on to the bridge. This puts more weight on the bridge - all that queuing traffic - and means the bridge has to be stronger to handle the weight. Stronger means more steel. More steel means more money. This means higher tolls. Think about it.

So they made the decision to let bikes through for free. This helps keep the traffic flowing at the toll. Less queues, less strain, less steel, less money. Everybody wins, but the greatest winner of all is Common Sense - sat in the corner proudly wearing his victory medal.

Of course, there decision could always have been made to charge people before they crossed the bridge - and traffic queues would then have been on terra firma - but maybe that was just too sensible. It is far more likely that a clever economist calculated that more vehicles would travel from England to Wales than the other way round (so they make more revenue by only charging for that direction). It is equally likely that the money had to be collected in Wales because it would then go into the correct state coffers. You and I will probably never know the details, but they will be along those lines.

This is an insight that not a lot of people seem to take the trouble to develop. A very simple truth which most of us choose to ignore: Nothing happens for no reason. In the world of man, everything happens for a reason. Everything is the process of the rational thought (blogs passim) of somebody. The things that you just accept are how they are - like where the toll booths of the Severn Bridge are placed - are actually the result of somebody making a deliberate decision. Think about that for a second.

When you realise this, you can take things apart to see them for what they are, and you can appreciate some of the thought that went into them (which is not always rational but appeared rational to somebody).

Take the example of a Give Way sign on the road. It's a triangle. It is also the only sign on UK roads that is an upside-down triangle. Doesn't that strike you as odd? Did you ever wonder why this was the case, or did you just accept it? There is a very simple reason why these signs are an upside down triangle: even if the sign is obscured (e.g. by mud or snow) then you can still tell what it is. Stop signs - these are octagonal rather than round for exactly the same reason. Nothing happens for no reason.

So maybe I think too much. In the words of Jarvis Cocker - "... stop asking questions that don't matter anyway". Maybe I should just accept that things are how they are, and stop melting my brain by processing all of this stuff. Perhaps my life would be far more content if I just took for granted that you don't wear hi-viz clothing in Wetherspoon's pubs and not worry about why that would be the case. But I don't have that choice - I am bipolar. I can't stop myself thinking about this stuff any more than I can stop myself blinking or breathing. Depending on where I am in the cycle, this affects the stuff I think about - but the clarity of thinking is always the same.

Back on a bike off-road tomorrow for the BMW Level 2 course. The key lesson that I want to get - and the one I am most afraid of - is "riding ruts and changing lines". I am told by previous participants that this consists of riding along a track with several ruts (normally car tyre ruts) towards Zippy who is standing in the road. He jumps from rut to rut and, as he does so, you need to get your bike into the rut that he is not standing in. Rinse repeat, as you ride towards him. The rule of the exercise is, apparently, "Don't run Zippy over". I am told that nobody makes it without falling off several times. Well, if something is worth doing then it's worth doing badly at first I suppose.

Given the rain that there has been in the West country, I think that Carnage is waiting patiently for his opportunity to have some fun at my expense. That said, the instruction at BMW is brilliant - you are never asked to do anything you have not been shown how to do. More importantly, you are never asked to do anything that you haven't seen the instructors do first. You are only ever asked to do things that you know are possible.

The best thing about BMW is the way they show you the techniques. Start with something that you think is impossible (like changing ruts). Have a six-times Dakar veteran show you how to do it, and explain what the technique is. Your first reaction is "WOW!". Then, when it's explained, you feel exactly how you'd feel if a magician showed you how a trick was done - "but that's just plain common sense ..."

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