16 Week Old Puppy Barking at Strangers

Question
Six weeks ago we adopted a female Tibetan terrier mix puppy (10 weeks old
at adoption, now 16 weeks old). We live in a large city, and have worked hard
to get her over her fear of the city noise, people and other dogs through lots
of exposure and weekly puppy socialization class. We've seen definite
improvement, though she is still a bit reserved and cautious when playing
with other puppies or encountering dogs on the street.

Strangely though, despite being comfortable on busy streets with lots of
people, when we encounter someone on an empty street (typically a single
person - for some reason 2 or more people are okay), which usually happens
on our nighttime walk, she barks like crazy. We went to the beach for the first
time this weekend, and the same thing happened: when one person would
walk down the empty beach, she would bark and bark at them. When we
reached a more populated part of the beach, no problem and no barking.

Does this sound like fear or protective barking? Is it normal that she's alright
around many people, but one person irritates her? She is becoming more
vocal, so could she just be trying out her new bark? It doesn't seem really
aggressive to me (no growling or anything), but it is incredibly frustrating and
stressful, and the last thing we want is for her to be the kind of dog that
scares people.

If you have any ideas as to what might be causing this, and how we can
prevent this from becoming a set behavior, we would greatly appreciate it.
Thank you!

Answer
It does sound like she's being protective. Distraction methods work great with cases like this. Here's what you do, when she begins to react, stop and stand right in front of her with a treat in hand to distract her. Keep her eye on it and use different noises like clapping or stomping to keep her attention. When and only when you have her complete attention, you can reward her. This works as it shows that not everyone is a threat.

You never know, she may have had an encounter with a single person before you got her and that could contribute to this. And anyone you feel comfortable with just walking, strike up a conversation and offer them to give her a treat as well.