Adam argues that because reflecting posts give drivers a clear view of the edges of the road, allowing them to drive more safely, marking road edges with such posts will decrease the yearly number of road accidents. Aiesha responds by pointing out something Mark fails to mention: that drivers speed more often and drive closer to the road’s edges on roads marked with reflecting posts. Both these behaviors tend to cause road accidents.

Aiesha undermines Adam's claim that the effect of marking the edges of the road with reflecting posts will be to decrease the number of annual accidents. While Aiesha doesn't question that reflecting posts help drivers see the edge of the road and make it possible for them to drive more safely, as Mark claims, she points out that in practice, drivers actually drive more dangerously on roads with reflecting posts, engaging in behaviors that tend to cause accidents. This suggests that the overall effect of adding reflective posts might not be to reduce road accidents after all.

Notice that this additional information weakens Adam's claim, but doesn't necessarily defeat it. It's possible that even though drivers engage in riskier behaviors on these roads, the increased visibility is enough of a benefit that there will still be fewer accidents, despite the riskier behaviors. But as this stimulus stands, Aiesha does effectively undermine Adam's argument with this additional information.