Learn Spanish (Latin America): Este es el tramo en el cual debes manejar lento. - This is the stretch in which you must drive slowly.

Both en el cual and en el que mean "in which" and are grammatically correct here. They both agree in gender and number with the masculine singular noun el tramo. The difference is mostly about tone: el cual sounds a bit more formal or written, while el que is more common in everyday conversation.

Could we just use que after the preposition, making it en que?

In modern Spanish, it is much more common and clearer to include the article, so we say en el que or en el cual. Dropping the article to say en que is sometimes seen in older texts or time-related expressions, but when referring to a specific physical noun like el tramo, you should stick to using the article plus the relative pronoun.

Can I use donde instead of en el cual in this sentence?

Yes, absolutely! Because el tramo is a physical location (a stretch of road), you can replace en el cual with donde. Saying el tramo donde debes manejar lento is very natural and often preferred in spoken Spanish.

Why is lento used here instead of the adverb lentamente?