zhuǎnwān yǐqián yào jiǎnsù shāchē.

Yes—here 以前 means before, and in Chinese it often comes after the event it refers to.

So:

• 转弯以前 = before turning
• literally: turn + before

This is a very common pattern:

• 吃饭以前 = before eating
• 睡觉以前 = before sleeping
• 上课以前 = before class starts / before attending class

So even though English says before turning, Chinese naturally says turning before.

Yes. 转弯前 is a shorter and very common version of 转弯以前.

• 转弯以前 = before turning
• 转弯前 = before the turn / before turning

In everyday speech and especially on road signs or instructions, 前 is often preferred because it is shorter and more compact.

For example:

• 过马路以前 / 过马路前 = before crossing the road
• 下车以前 / 下车前 = before getting off

Both are correct here.

Here 要 means something like need to, should, or must depending on context.

In this traffic-safety sentence, it is not really about personal desire (want to). It expresses necessity or instruction:

• 要减速刹车 = need to / should / must slow down and brake

So in this context, 要 is closer to:

• You should...
• You need to...
• You must...

A learner should be careful because 要 is very flexible:

• 我要咖啡。 = I want coffee.
• 我要去学校。 = I am going to school / I want to go to school.
• 转弯以前要减速刹车。 = Before turning, you should/must slow down and brake.

Chinese often omits the subject when it is obvious from context.

In a traffic instruction like this, the implied subject is something like:

• 你 = you
• 驾驶员 = the driver
• or anyone operating a vehicle

So the full idea is:

• （你）转弯以前要减速刹车。
• Before turning, (you/the driver) should slow down and brake.

This kind of omission is very natural in Chinese, especially in:

• instructions
• signs
• warnings
• general statements

Chinese often places verbs one after another without an explicit word for and, especially when describing a sequence of closely related actions.

So:

• 减速刹车 = slow down and brake

This is understood as two connected actions:

1. 减速 = reduce speed / slow down
2. 刹车 = brake / apply the brakes

Chinese does not always need a connector like English and. Context makes the relationship clear.

In writing, you might also see punctuation added for clarity:

• 转弯以前要减速、刹车。

But even without punctuation, the meaning is still easy to understand.

They are related, but not identical.

• 减速 = to reduce speed, to slow down
• 刹车 = to brake, to apply the brakes

You can 减速 in different ways, not only by braking. For example, you might slow down by easing off the accelerator.
刹车 specifically refers to using the brakes.

So putting them together emphasizes safe driving behavior before a turn.

It can be both.

As a verb:

• 刹车！ = Brake!
• 开车时要及时刹车。 = When driving, you need to brake in time.

As a noun:

• 我的刹车坏了。 = My brakes are broken.
• 踩刹车。 = Step on the brake.

In your sentence, 刹车 is a verb.

In 转弯, 转 is pronounced zhuǎn, so the whole phrase is:

• 转弯 zhuǎnwān

The character 转 has more than one pronunciation, and the pronunciation depends on meaning.

Common ones include:

• zhuǎn — to turn, change direction, transfer
• zhuàn — to rotate, spin around

In 转弯 (to turn / go around a bend), the correct reading is zhuǎn.

So:

• 左转 = turn left
• 右转 = turn right
• 转弯 = make a turn / turn a corner

Yes, 以前 can also mean ago / previously, but the structure tells you which meaning it has.

Two common uses:

1. After a time word → ago

   • 三天以前 = three days ago

2. After an action/event → before

   • 出门以前 = before going out
   • 转弯以前 = before turning

In your sentence, 以前 follows the action 转弯, so it clearly means before.

That is also grammatical. Adding 在 can make the time phrase feel more explicit:

• 在转弯以前，要减速刹车。

However, Chinese often leaves out 在 when the time phrase is already clear, especially in short instructions or sign-like language.

So:

• 转弯以前要减速刹车。 = natural
• 在转弯以前要减速刹车。 = also natural, slightly fuller

The version without 在 feels especially suitable for warnings and concise guidance.

It sounds a bit like instructional / safety language, which is very normal for traffic rules, driver training, or warning signs.

In everyday conversation, a person might say things a little differently, for example:

• 转弯前要先减速。 = Before turning, you need to slow down first.
• 拐弯的时候要踩刹车。 = When turning, you need to brake.
• 转弯前记得减速。 = Remember to slow down before turning.

So your sentence is perfectly natural, but it has a concise, rule-like tone.

Yes, depending on context.

As a verb, it means:

• to turn
• to make a turn

As part of road-related language, it can also refer to the idea of a turn / bend in the road.

In your sentence, 转弯以前 is best understood as before turning or before a turn. English can express it either way, and Chinese allows that same flexibility.

Not necessarily. Chinese often omits punctuation between closely connected verbs.

Still, depending on style, you might also see:

• 转弯以前要减速、刹车。
• 转弯以前要减速，刹车。

The version with 、 neatly separates two parallel actions.
The version with no punctuation is also common in short instructional text.

So the original sentence is acceptable and easy to understand.