I remember reading in the excellent book Schott's Miscellany, of the weight bearing properties of different ice thicknesses. Thankfully someone on the UK hillwalking forums has transcribed it - thank you whoever Ron is:

Theoretically 'safe' thickness of ice for different weights; from Schotts original miscellany, Ben Schott. Not sure exactly how it was calculated, it assumes solid blue/black ice.
all values in inches

single person on skis 1.5
single person on foot 2.5
Group in single file 3
Snowmobile 3
Average car 7.5
Large car 8
Medium truck3.5 tons 9
7tons 10
15 tons 15
25 tons 20
45 tons 25
70 tons 30
I am not sure whether these values are factored.
Don't blame me if it isn't right.
Ron