Translation:The safest route is the one on the left.
It could mean either out of context. Generally, when you have no context, the absolute superlative is assumed.
This is quite similar to assuming "it" as opposed to he/she when something is out of context. For example, consider «Es excelente» out of context. The most common translation is "It's excellent." However, consider the following context «¿Es buena trabajadora? Es excelente». The translation given the context is now "She's excellent."
In English, "safest" is a superlative, and "safer" is a comparative. Same for "most" (superlative) and "more" (comparative). From what you are saying, I gather that the distinction between superlative and comparative in Spanish can only be determined from context.
Am I understanding this correctly?
It is es because it's saying that something is always true. The safest way is always on the left in this case, and that's a fact. Although esta' is used for location too, it is also used for temporary situations. If you used esta' in this sentence, you'd be saying that sometimes the safest route is that on the left, but other times it's a different way. I hope this was helpful to you.