"La ruta más segura es la de la izquierda."
Translation:The safest route is the one on the left.
"to the left" = "a la izquierda."
"es la de la izquierda" = "is the one on the left." While its true that you could probably use both in a situation, "is the one on the left" and "to the left" are two different things.
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?
I believe location would be the key part if it was "La ruta más segura está a la izquierda". Here you don't say "the safest route is on the left", but "the safest route is the one on the left".
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.
I think it should have accepted "that of the left", but I guess maybe it's technically not proper English to say "of the left" in this case. "That on the left" makes sense as well, so I'll give DL the benefit of the doubt.