The correct answer is none.
The vast majority of runners do at least some running on or near roads. Anywhere moving vehicles and people mix, collisions are sure to happen. Runners are seldom hit from the front, since the eyes can provide enough warning to avoid a collision.
Most runners get hit from the back or side, where the ears are the main warning source. Intentionally reducing ear sensitivity while running anywhere near traffic is literally suicidal. Many communities understand this, and have passed laws restricting the use of earphones and music players near roads.
I have personal experience with this: Just over 25 years ago a runner wearing earphones was waiting for the light to change at an intersection, then proceeded to run across the wrong side of the intersection. I was just entering the intersection on my motorcycle, having timed the light perfectly, only to suddenly find that runner in front of me.
I hit the brakes and horn and veered to miss the runner, but she kept going, never hearing the huge amount of noise my horn and tires were making. I was unable to avoid slamming into her. I had a very rough landing, was knocked unconscious, and nearly slid into oncoming traffic.
When I woke in the hospital, two police officers were standing at the foot of my bed. They asked if I knew what had happened, and I told them everything I could remember. My memory abruptly ended an instant before the impact. I didn't remember the collision itself or anything after.
They next told me she was declared dead at the scene. My blood pressure crashed and I passed out for a few moments. When I came to again, they said something that's been burned into my memory ever since: "It was not your fault. The witnesses and the evidence at the scene make it clear you did everything possible to prevent the collision. The earphones she was wearing and the volume setting of her music player combined to make her completely unaware the danger she was in. She was negligent to the point that she essentially committed suicide, and used your motorcycle to do so." They said more after that, but my mind had locked up trying to process that last sentence.
Even now, a quarter of a century later, this memory still wakes me, my heart thumping and my hands shaking.
There is no safe way to combine music and traffic with running or bicycling. Just being a runner or bicyclist on a road is hazardous enough without making it worse by adding music.
The music a road runner hears is often their own requiem.
I'm enough of a personal libertarian to believe that we each have the right to determine when and how we leave this world. I also believe in personal and social responsibility, and we should not inflict needless trauma on others.
Wearing earphones while running or biking anywhere near a road is equivalent to intentionally making yourself a candidate for a Darwin Award. But as you exit the gene pool, you should try to do so with minimal pain to loved ones and strangers alike.
Wearing only one earphone or keeping the volume down is not a viable solution: Your attention will still be on the music, instead of on the hazards present in the world around you.
The same applies to using a phone while running.
If you must run with music, please stay well away from traffic, and consider these alternatives to roads and sidewalks:
- Treadmills
- Oval tracks
- Paved paths
- Trails
Update, 16 August: I've received a fair amount of feedback about this post. Some said they rely on music to keep their pace regular. In that case, consider running with a metronome such as the inexpensive Seiko DM50. The beep of the metronome will not prevent you from hearing approaching traffic. I seldom run without mine!
