Everyone has pretty much covered it very well. You will feel like a goober, but at your local shop wear the helmets for about 15 minutes each to at least allow the padding to settle somewhat and look for hot spots where you feel pressure - especially in the forhead area. Measuring your head is a good idea, but not the end all. You must determine your head shape - oval, round long oval etc. Each mnfr uses different molds for each model type - ie the Arai profile is more long oval than the Astral X used to be.
This is just MHO, and I haven't tried thousands of helmets, but I do think the Shoei, Arai, Suomy - while being way more expensive tend to flow air better while still giving a quiet ride. I ride with earplugs anyway so the noise is never a factor, but My Shoei and Arai seem to have the best airflow and comfort when hot - HJC and Scorpion are good, but not as good. Also the more expensive models tend be be less weight - several onces may not seem like much, but over a long day can really weigh down your neck.
Refering to the impact study - it is all about energy transfer and being an engineer the study made some very reasonable points having to do with lid construction and energy absorbtion rates - definitly worth the read.
Let me also encourage you to actually buy your helmet from your dealer. If you go in and get good help from someone who knows how to fit a helmet and provides you with helpful advice, buy it from the shop and reward their expertise with that extra $30 or so. If you get no help or worse a 19 year old squid who knows nothing more how to stand a 600 on end, but can't get around a corner to save his life - by all means try all and find the best fit then buy online. The reason I say this is because at the shop I frequent, there are two guys who work there who will spend all the time you need answering questions, recomemding things etc. They have been riding for years and are eager to help others enjoy what they love. These type of shops and workers are deserving of that extra you can save online. Nuf said. Comfort and fit is king all else is secondary - enjoy your new lid.
Here is the link
http://www.motorcyclistonline.com/ge...helmet_review/