This JAMA article examines the health value of physical activity and especially riding your bike to work.  Not only is cycling good for your health, it is good for the environment and a great way to get around and add more enjoyment to your day.  The article concludes that people that ride their bicycles to work have 40% less all cause mortality.

This June 12, 2000 JAMA article examines physical activity during leisure time.  Leisure time and physical activity was inversely associated with all-cause mortality in both men and women in all age groups. Benefit was found from moderate leisure time physical activity, [with higher] benefit from sports activity and bicycling as transportation

Physical activity was assessed by self-report, and health status, including blood pressure, total cholesterol level, triglyceride levels, body mass index, smoking, and educational level, was evaluated.Main Outcome Measure  All-cause mortality.Results  A total of 2881 women and 5668 men died. Compared with the sedentary, age- and sex-adjusted mortality rates in leisure time physical activity groups 2 to 4 were 0.68 (95% confidence interval, 0.64-0.71), 0.61 (95% confidence interval, 0.57-0.66), and 0.53 (95% confidence interval, 0.41-0.68), respectively, with no difference between sexes and age groups. Within the moderately and highly active persons, sports participants experienced only half the mortality of nonparticipants.

Bicycling to work decreased risk of mortality in approximately 40% after multivariate adjustment, including leisure time physical activity.

Source: All-Cause Mortality Associated With Physical Activity During Leisure Time, Work, Sports, and Cycling to Work | Physical Activity | JAMA Internal Medicine | The JAMA Network