Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Ed, you've got the point slightly wrong. Cycle trainers want to encourage people to look more rather than listen. If their hearing is blocked, they can't help but look more. Now, while it is possible to conduct an experiment on this by blocking someone's hearing, it is still not desirable to obstruct one of the senses. The point is just that looking is far more important than relying on hearing.

This.

In my opinion (based on my own experience) too many pedestrians and cyclists rely on hearing in favour of proper observation. I'm always surprised at how many people I see on my commute who pull out to overtake another cyclist (or a parked car) without even looking behind them. Most cyclists are near silent, and more and more cars on the road are electric/hybrid that are near silent.

Mind you, some of the people riding like twats and pulling out without listening have got headphones on; so it's not as if it is a magic pill to make people look behind more.

Wearing headphones whilst cycling certainly made me realise I wasn't doing enough rear observations (I thought it had been drummed into me from years of riding a motorbike*) as I often surprised by something coming past me that I hadn't seen. I'm currently in a phase of not wearing headphones whilst commuting, I may need to try it again soon to see how I'm doing.

Some people obviously don't have a problem with such a lack of awareness.

• and I always wore earplugs when riding a motorbike as the wind noise at 70mph (and above) is bloody loud.