Cautions and Yellow-Flag Lead Changes

Lead changes are frequently used as a way to measure how ‘good’ the racing is. But yellow-flag lead changes aren’t all that exciting. Even worse, they skew the statistics because a race with a lot of cautions will likely have a lot of lead changes.

In trying to evaluate the impact of the 2019 rules package, I hypothesized that what we really should count are green-flag lead changes, but before I went to all the trouble of separating them out, I wanted to be sure that it would really make a difference.

Hypothesis

The more cautions, the more yellow-flag lead changes there will be. A race that happens to have a lot of cautions might look like a better race because it has more lead changes, but actually, the majority of the lead changes take place off the track.

Test

I plotted yellow-flag lead changes as a function of the percentage of the race run under caution (which is essentially caution laps corrected for the track length). I used percentage because otherwise races of different lengths end up being weighted differently.

A scatter plot showing a correlation between the percentage of the race run under caution and the number of yellow-flag lead changes
Data are taken from races from 2010-2019 at tracks from Atlanta to Dover alphabetically, excluding Daytona.

I plotted data for racetracks alphabetically up to Dover (excluding Daytona) and stopped there because I think it’s clear that there is a trend.

It’s not a simple correlation because the length of each caution determines whether it’s possible to have more than one lead change as people pit. A quickie caution may not allow time for any lead changes. While you may one or two lead changes during a six-lap caution, that doesn’t mean you’ll have three or four during a 12-lap caution.

Conclusion

More cautions mean more yellow-flag lead changes. Yellow-flag lead changes aren’t what we’d call ‘good racing’, so what we ought to be comparing are green-flag changes.

The number of cautions can have a big impact on the number of ‘quality’ lead changes, so it’s worth the trouble to separate them out.

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