That is unusual - most places go by momentum. Hence, White Van Man.

Predatory drivers in the Bay State know that White Van Man generally has higher and more lucrative liability insurance than they do. White Van Man knows it, too. So they keep it simple, mass-wise.

IIRC, the rules I saw in Germany were more complex, but mass was still not a dominant factor. Something like

Roundabout ROW = (velocity*(sqrt(vehicle mass)) / (vehicle value in Euros)

Again, however, if eye contact is initiated, that driver's velocity is set to zero for purposes of the calculation.