Heard a story...

I heard a story from a fellow cyclist the other day of him racing another cyclist through London. They were head to head pretty much the whole way from Euston to Elephant & Castle... a battle of perseverance between the second-hand mountain bike and the flash new road bike. It resulted in a triumphant victory of the mountain bike, showing that its not all about what kind of bike you have but the level of fitness and sheer stubbornness of not being overtake by other traffic.

"Sprint", Tour de France, sportdw.com 

So how is this at all relevant?
It reminded me that traffic is not just about cars, buses and trucks. I personally feel a lot more nervous around other cyclists than I do around buses, because they are just as unpredictable as me.
 Buses from my experience are quite good at using their indicator to show switching lanes and pulling away from bus stops.
Many cyclists however, wait at lights all over the place,cut across lanes without indicating and squeeze through small gaps to get ahead of the queue. Not to mention the swerving as we set off from the lights and catch our balance.
Cyclists seem to squeeze together a lot more than cars would, yet are a lot less predictable than cars.

Cyclists at lights (i-sustain.com)

With the rise of cyclists in London, due to the increased use of "Barclays Bikes" (better known as Boris bikes) and the encouragement of staying healthy, the roads are getting busier and there is a rise of new/occasional cyclists who haven't quite got the "cycle ettiquate" allowing the 2-wheeled traffic to flow smoothly.

So stressful/agitating journeys could be due to many commuters on bikes at peak times, not just high levels of traffic.