It's always worth having two.... as I discovered on Friday, (luckily in the gloom rather than the dark), that my rear light looked fine when I put it on the bike, but at the far end it was off (stupidly!!! I didn't actually think to see that it was off rather than batteries ran out, just assumed I had failed to turn it on), on the return journey I made sure it was on as I set off and sure enough it was off at the other end. At least this has now reminded me to go and replace the batteries which I failed to do on Friday!!!