24 Hours of Tri - Bike slowdown at night? How much?

Hi -
We're doing a bit of planning for our 24HOT team. We did 8 hours last year, so we're familiar with the course, but we got to stop biking before dark

We figured out yesterday that there's pretty much no way to get around biking at night, at least a few laps.

I was wondering if anyone from last year has any scoop on about how much their biking slowed down at night bc of the dark (yes, of course, we'll have headlamps and lights and such).

I was thinking 30%? But I'm not sure it would even be that much if we're only factoring in dark (not fatigue, we've already got that factor in...). I recall the road being in pretty good shape, but the only night biking I've done is on a mtn bike, going much slower, so I'm not sure how likely it is that my bike light will be strong enough to ride fast & still be able to see far enough ahead.


Thanks....