  From a recently found family of analytic, finite and accelerating
1+1-dimensional solutions to perfect fluid relativistic hydrodynamics, we
derive simple and powerful formulae to describe the rapidity and pseudorapidity
density distributions. By introducing a new scaling function, we notice that
the rapidity distribution data of the different experiments all collapse into a
single curve. This data-collapsing (or scaling) behaviour in the rapidity
distributions suggests that high-energy $p+p$ collisions may be described as
collective systems.
