What kind of ID seems to be a repetitive subject. Maybe the first things to think of is When and Where you would need ID?
First, would be if you were stopped for some reason by law enforcement and needed to prove who you are. Pretty unusual in this day and age for anyone to take your word for your identity. Depending on circumstances nothing short of government issued picture ID will work. If you live in a state that issues a State ID carryiing it with you solves the problem
Second, would be where you were injured and were unable to to respond to First Responders and/or Emergency Medial Care. There a simple dog tag with name, Social Security number and maybe a contact phone number is ideal. In that situation, from experience, notifying relatives is way down the priority list.
Where to carry? From long experience the military requires dog tags to be on the body. A chain around the neck is best. Putting ID Tag in a pocket, shoe, or any other place just does not do the job. Of course, if you are involved in a law enforcement stop you will be able to talk so putting your picure ID in a pocket is probably OK for most situatiions.
Road ID, as has been posted, has done a good marketing job. But, their products don't do either job very well and are expensive.