http://www.cyclehelmets.org/1139.html

Cycle helmets are specified by their manufacturers as meeting one or more of the international standards for this equipment. All of the standards test the helmet's protection of only a decapitated headform, (i.e. one with no body attached); and all tests involve only low speed impacts. Impact speeds are less than 6.6 m/s (24 km/h or 15 mph), and in some cases, barely 5 m/s (18 km/h or 11 mph). Unlike seatbelt tests, helmet test standards do not realistically replicate serious crashes.

The highest standards are those of the nominally independent Snell Foundation, of which the B-95 standard is the most rigorous; however, the Snell standard is rarely used by manufacturers. The European standard EN 1078 is particularly weak and some other standards have declined over time. This is in part a reflection of the involvement in these standards committees of the helmet manufacturers, who want to minimise production costs in a fiercely competitive market. Few helmets are designed to offer protection better than that specified by the standards.

There is no mandatory, third-party testing of helmets, and independent surveys have shown that many of the helmets on sale do not meet the standards to which they are accredited. Very few meet the more demanding standards.