I dunno -- SM had a pretty spendy bike and I don't see her scrimping on the helmet. Wouldn't be surprised if her helmet was above the standard. In other words, the hard shell is built to protect yet slide on impact.
The Ideal Bicycle Helmet
Standards in the US have evolved over more than two decades since the original ANSI Z90.4-1984 standard was adopted in 1984. ANSI let its standard lapse rather than update it, and the ASTM bicycle helmet standard took its place for some years. Now all helmets manufactured for the US market after 1999 must by law meet the Consumer Product Safety Commission's standard (CPSC). So you should be looking for a helmet with a sticker inside that says it meets the CPSC standard. There are also the
Snell Foundation's bicycle helmet standards. Some manufacturers are still using Snell's older B-90 standard (introduced in 1990) which is comparable to CPSC. But Snell's newer B-95 standard is the most stringent standard in the market -- more difficult to meet than the ASTM or CPSC standards -- and ensures that the helmet has been certified by the independent Snell Foundation labs. We would prefer a Snell B-95 helmet, but we would not worry much about the differences between that and a CPSC helmet. And some day we would like to see the CPSC standard improved to require more coverage and a softer landing.
5. Outer Shell
In a crash you want an interface with the road that is smooth, hard, round and slick. That keeps your head from snagging, which can add to the severity of the impact and may even jerk your neck. The elongated "aero" style has a tail on the rear than could shove the helmet aside when you hit, leaving your head unprotected.
Professor Hugh Hurt raised this question again, based on both testing problems and field reports of injury from helmets being pushed aside.
There are very few hard shell helmets like the 1970's models, primarily because they were so heavy and expensive to manufacture. Only one company makes them today in true ventilated bicycle styles, Hopus Technologies, and their
updated models show what you can still do with ABS hard shells. The thin shells on today's market are nearly as good in sliding resistance, and some of them are evolving into slightly harder shells now as the manufacturers try to open up more vent area and rely more on the shell for impact strength. Vents are necessary, but must be smoothly faired into the helmet shell. We advise consumers to avoid any helmet with unnecessary fashion ridges on the outside, or protruding snaps for visors, or any other feature that could cause the shell to snag. This is an easy item for a consumer to assess, as long as you keep in mind that you want your head to slide on impact. We hope to amend the ASTM standard some day to add a requirement to measure sliding resistance of the shell. We have put up
the lab study that established the value of a round, smooth, slick outer surface if you want to see the scientific data.