a headlight a kilometer away may be spread over 36000 square meters including your face. (more detailed info on headlight antenna gain would be appreciated.) a 1200-lumen high beam over that area is 0.03 lux
a 500 nit phone screen emitting over about a steradian from an 80mm × 160mm area is 6.4 lumens, but at night you usually turn it down to minimum brightness, say 0.6 lumens. (i haven't measured.) at a distance of 300mm that steradian is 0.09m², so it's about 6 lux
so the cellphone is about 200 times brighter than the headlight at that distance, and that's also observable from looking at people using cellphones walking on the highway
but the person i was asking for more detail didn't specify that the headlight is pointed at your face; and in most situations where you can see headlights, they're not pointed at you. that's just a detail you filled in
a 500 nit phone screen emitting over about a steradian from an 80mm × 160mm area is 6.4 lumens, but at night you usually turn it down to minimum brightness, say 0.6 lumens. (i haven't measured.) at a distance of 300mm that steradian is 0.09m², so it's about 6 lux
so the cellphone is about 200 times brighter than the headlight at that distance, and that's also observable from looking at people using cellphones walking on the highway
but the person i was asking for more detail didn't specify that the headlight is pointed at your face; and in most situations where you can see headlights, they're not pointed at you. that's just a detail you filled in