due to growth function in two branches, but only limited in gap (12.8%), the total already becomes more than 100% in very early years.
Please note, growth of vehicle population and growth of share does not imply same thing.
To avoid this, i would recommend to calculate the verticals number explicitly and then convert them to share and instead of growth, use interp function, then you can use remainder function as well, to avoid error that can arise due to tiny decimal numbers.