Visibility from a tank is not that good, so they may legitimately not notice the mine. If you want to actually hide an anti-tank mine, it's also possible to dig the mine in a hole and put a layer of soil on top. But even if the tank crew notices the mine, it's still not completely useless, since the mine is still blocking the route for the tank and preventing its advance.

The small window below his head is what they have to look through when they can't have their head out. T-72

Even if they spot it on roads, it would still be blocking way. You'd expect mines in the grass near the road too then.