My guess is that loss of individuals to vehicle strike is not important for the overall population. What's important is the carrying capacity of the habitat which, according to this article, has been significantly increased.
It's likely as nothing compared to running over the strip with a tractor / slasher twice a year ... BUT I expect the timings of those ensure minimal impact to the life cycles of the insects, plus most that are in a mobile phase of their life will likely fly out of the way quickly enough.
Either way, the fact numbers are increasingly rapidly suggests the tradeoff is in favour of the butterflies.
Most butterflies will mate, make territories and remain in the flowered area. As they have yet all that they want, the need to move on and cross the road will decrease. Many butterflies will lay in weeds also so the area is safe for they for most of this live cycle. And in winter they simply will migrate. Is much better than the other option, safer for bikers and cheaper for the government.