The Proper Fit
According to the CPSC, bicycle helmets “should be both comfortable and snug.” When the helmet is strapped on your head, it should not move much back to front or side to side. It should sit straight on your head, parallel to the ground. You should not be able to pull the helmet off your head, even with vigorous effort.
Other Considerations
Safety rating and fit are not the only concerns in choosing a bike helmet. Darker helmets may be both harder to see and hotter to wear. Helmets with too many vents can be noisy or detract from crash performance. Helmets with too few vents can leave you hot and sweaty, and potentially deter you from wearing them. The straps on the helmet should be comfortable, strong and reasonably easy to use.
There is undeniably an element of fashion to bike helmets. While personal style is certainly a consideration, it should be secondary to the safety of the helmet. When choosing between two equally safe options, style is a fine tiebreaker.
Many bike shops can assist you in selecting a helmet that will serve your interests. If you have the chance, ask for help in finding the best fitting option. Anything that encourages you to wear the helmet every time you get on your bike is a good thing.
Source: Helmets.org, “A Buyer’s Guide to Bicycle Helmets“
Source: United States Consumer Product Safety Commission, “Which Helmet for Which Activity?”