  The deuteron coalescence parameter $B_2$ in proton+proton and nucleus+nucleus
collisions in the energy range of $\sqrt{s_{NN}}=$ 900 - 7000 GeV for
proton+proton and $\sqrt{s_{NN}}=$ 2 - 2760 GeV for nucleus+nucleus collisions
is analyzed with the Ultrarelativistic Quantum Molecular Dynamics (UrQMD)
transport model, supplemented by an event-by-event phase space coalescence
model for deuteron and anti-deuteron production. The results are compared to
data by the E866, E877, PHENIX, STAR and ALICE experiments. The $B_2$ values
are calculated from the final spectra of protons and deuterons. At lower
energies, $\sqrt{s_{NN}}\leq 20$ GeV, $B_2$ drops drastically with increasing
energy. The calculations confirm that this is due to the increasing freeze-out
volume reflected in $B_2\sim 1/V$. At higher energies, $\sqrt{s_{NN}}\geq 20$
GeV, $B_2$ saturates at a constant level. This qualitative change and the
vanishing of the volume suppression is shown to be due to the development of
strong radial flow with increasing energy. The flow leads to strong
space-momentum correlations which counteract the volume effect.
