dào lùkǒu yào jiǎnsù.

Here 到 means when reaching / upon arriving at / when approaching a place.

So 到路口要减速 is not just a dictionary-style arrive. In this sentence, 到路口 sets the situation: when you get to the intersection or as you come to the intersection.

In traffic language, English often sounds more natural as Approaching an intersection, you should slow down.

Chinese often leaves out the subject when it is obvious from context.

In a traffic instruction like 到路口要减速, the understood subject is something like:

• 你 = you
• 车辆 = vehicles
• 驾驶员 = drivers

So the sentence is basically saying:

• You should slow down when you reach an intersection
• or Vehicles should slow down at intersections

Omitting the subject makes the sentence sound like a general rule or instruction.

Here 要 does not mean want to.

It means something closer to:

• need to
• should
• must
  depending on context

In instructions, warnings, and rules, 要 often expresses necessity or requirement.

So here:

• 到路口要减速 = When you reach an intersection, you need to slow down
• or more naturally, You should slow down at an intersection

If you translated 要 as want to here, it would sound wrong.

Because 到路口 is functioning as a time/place condition: when you get to the intersection.

Chinese often puts this kind of condition first, then gives the main action:

• 到路口 = when reaching the intersection
• 要减速 = should slow down

So the structure is:

condition/situation + main statement

If you said 要到路口减速, it could sound like need to go to the intersection to slow down, which is not the intended meaning.

Yes. 到了路口要减速 is also natural.

The difference is subtle:

• 到路口要减速 = a slightly more general, sign-like, concise instruction
• 到了路口要减速 = explicitly when you have reached the intersection

On signs, rules, and short instructions, Chinese often prefers the shorter version without 了.

So both are understandable, but 到路口要减速 feels especially suitable for a traffic sign or brief reminder.

Because 到路口 and 在路口 do different things.

• 到路口 = when reaching the intersection / on arriving at the intersection
• 在路口 = at the intersection

This sentence focuses on the moment of approaching or reaching the intersection, so 到 is the better choice.

Compare:

• 到路口要减速 = Slow down when you get to the intersection
• 在路口要减速 = At the intersection, you should slow down

The second one is not impossible, but the first one better matches the idea of approaching a junction and reducing speed.

路口 means a road junction, intersection, or crossroads.

Breakdown:

• 路 = road
• 口 = opening, mouth, entrance

So 路口 is literally something like the opening/junction of roads.

It is a very common word in traffic contexts. Depending on the situation, it may refer to:

• an intersection
• a crossing
• a junction

Yes. 减速 is a verb meaning to slow down or to reduce speed.

Breakdown:

• 减 = reduce, decrease
• 速 = speed

So 减速 literally means reduce speed.

It is especially common in formal, written, or traffic-related language.

You may also hear more conversational alternatives such as:

• 慢一点 = go a bit slower
• 开慢一点 = drive a bit slower
• 放慢速度 = reduce speed / slow down

But on signs and in rules, 减速 is very standard.

It is fairly concise and somewhat formal, which is exactly what you would expect for traffic instructions, rules, or signs.

It sounds natural in contexts like:

• road signs
• driving manuals
• safety instructions
• official reminders

In everyday conversation, someone might say something a bit fuller, such as:

• 到路口的时候要减速。
• 快到路口了，减速。
• 前面路口，慢一点。

So the given sentence is natural, but it has the compact style typical of written instructions.

A useful way to read it is:

• 到路口 = when you get to the intersection
• 要 = should / need to
• 减速 = slow down

So the full structure is:

when X happens, Y should be done

More generally, this pattern is common in Chinese:

• 到地方要打电话。 = When you get there, you should call.
• 到学校要先登记。 = When you arrive at the school, you need to register first.

So 到 + place + 要 + action is a very useful pattern to learn.