I think it's causal. The cruise control on my car is busted and I don't really feel like shelling out four figures to fix it, so I drive without. I also speed probably 10-15 over pretty often when I'm on long stretches of highway. Of course, I'm also in Texas where this is fairly common and poses less of a risk than eg in Virginia.

The funny thing is I might actually be safer without it, as it's the old static-speed cruise control not adaptive. While I'm less patient to idle along at 75, I am also more attentive. Who knows.