Do plants have consciousness?
Are they "sentient beings?

Lama Thubten Shenphen (May 2002): Plants do not hold a consciousness, an individual
consciousness. So when you cut a salad, a flower, a plant, you do not kill a being. This should be clear. Only in some particular cases, that some big trees have a spirit consciousness [Skt. yaksha] in them. In such case, it's not a being that has taken rebirth as a tree, but as a spirit who will be 'linked' into a tree, as it could be into a house, a rock, or whatever else. The vegetal realm does not exist as a possible realm of rebirth. Thus, to cut a plant is not killing, by any way.

The second question implies the insects in the salad; well, we have to be careful as much as we can. The vow of not killing, in its major form, does not include insects or animals, but include human beings. Therefore, clearly, if you kill animals, having any vows or not having any vows, it is a negative karma, but it is not breaking the vow. Otherwise, if you have the Bodhisattva vows, you could not take a car any more.

We have to be careful as much as we can, and on the road, when there are frogs, or when there are animals, we have to drive more slowly, to be able to avoid them as well as we can. We cannot avoid them all, as we understand when we look at the front of the car.
If you can, and if you are not afraid of remarks of others, then you can stick a mantra in front of your car! Either you can buy an adhesive one, or you can make one, or ask somebody to make an 'Om mani peme hung' mantra and stick it on the front side of your car, as it is said that it is very valuable for the insects that will collapse on this part of the car.