Is sigmoidoscopy safe? is it true that it can detect the majority (70%) of colorectal cancers?

Yes: Unless you have a family history of colon cancer occurring at less than 60 years of age, in your family, you may wait till you are 50 for the first screening colonoscopy. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.

Yes : It is relatively more safe but will miss proximal colon lesions above the sigmoid. In my opinion Colonoscopy unless contraindicated.