As we know that centripetal acceleration is given as

so here we all know that radius of curvature of straight surface or flat surface is infinite or very large
So here acceleration will be zero when it move on flat surface
So here at points "w" , "x" , "z" the surface will be flat surface
and hence the observer will not experience centripetal acceleration
while at point Y the surface is curved and hence the observer will experience centripetal acceleration