New data from Consumer Product Safety Commission shows number of head injuries among bicycle riders has increased 10 percent since 1991, in period when bicycle helmet use has risen sharply and ridership has declined; given ridership decline, rate of head injuries per active cyclist has increased 51 percent just as bicycle helmets have become widespread; safety experts say helmets do not prevent accidents from happening, but are extremely effective at reducing severity of head injuries when they do occur; say helmets can reduce severity of brain injuries by as much as 88 percent; experts are mystified as to why injuries are on rise; some cycling advocates contend that rising numbers of aggressive drivers are at fault; others suggest that many riders wear helmets improperly; specialists in risk analysis say riders using helmets may feel inflated sense of security and take more risks; photos; chart (L)

University of Bath News and Information - providing access to information and news for staff and students at the University and press release articles to the media