I know this because I've measured them online, then gone out with my Garmin on to log my intervals. 800m one way, 800m the other way, and repeat; all because going to the running track makes interval training less convenient and more scary than doing it just outside my flat or office.
I like the idea of the running track - the lanes, the roundness of it and the feeling that, by running on it, you're a real runner. I have a fantasy of going down to the track, kit bag slung over my shoulder and doing all sorts of warm-up maneuvers that I never do when I run on the street, shedding various layers of clothing as I do. And then, when I'm sufficiently warm, I'll sprint round that oval in a pair of spikes I don't yet own and stab at the lap button on my watch.
But this has never happened because the running track costs £3 to use, is a mile from my house (ideal warm-up distance but completely ruining the warming up on the track fantasy element) and is full of other runners. I like other runners, I think they're great. But I don't like having them around when I'm trying to run fast.
Slower runners (though there aren't many slower than me at the track) mean having to overtake, making each lap a few painful meters longer. And, quite frankly, what's the point of paying to use an accurately measured 400m oval if you're going to run 402m?
Faster runners are annoying too. Here I am, giving myself a peptalk, telling myself "you've got this, you're a machine, you're going so fast, hang on in there" only for someone in a smaller pair of shorts than me to zoom past like I'm still in the starting blocks. I like to think of myself as some sort of Speedy Gonzalez while I'm busy trying not to cough up my lungs and would much rather not be reminded of the truth - it makes the reps easier to finish that way.
So instead I use roughly measured sections of pavement. I've dodged workmen, tourists and other runners, all the while reminding myself not to be sick. That's the secret to a good interval session - run hard enough that you feel a bit sick, but not so hard that you do make yourself chunder. It's a difficult balance to achieve which makes my spot on The Embankment all the more ideal - a discreet lean over The Thames and nobody's the wiser. Try doing that at the track.