Anecdotally, I&#x27;d say that something like 50% of the cars I see on the road are at least 10 years old. Probably 15% are 20 years old or more.  I live in one of the poorer states in the US, so my experience may be somewhat skewed.<p>This also brings up one of my concerns with self driving cars.  A vehicles (or and mechanical device) age, they become less reliable.  The brake booster on my truck went out today (20 years old).  I could still brake, but it took significant effort.  I wonder what the failure modes look like on the self driving apparatus, and wether they fail gracefully and safely.  I think it will be important that when we have 10 or 15 year old self driving vehicles on the road that they have engineered in safe failure processes.