Na ovoj cesti ne smiješ pretjecati autobus.

Because Croatian often puts the setting or context early in the sentence.

• na = on / at
• ovoj cesti = this road

So Na ovoj cesti means on this road and gives the location where the rule applies.

This word order is very natural in Croatian, especially in instructions, rules, and signs.

It is in the locative singular.

After na, when you mean location rather than movement, Croatian uses the locative:

• na ovoj cesti = on this road / on this street

Here there is no movement onto the road; the sentence is about something happening on it, so the locative is used.

Compare:

• na ovoj cesti = on this road (location, locative)
• na ovu cestu = onto this road (movement toward, accusative)

Because ovaj (this) has to agree with the noun it describes.

The noun cesta is:

• feminine
• singular
• here in the locative

So the correct form is:

• ovoj cesti

A few forms of ovaj with a feminine singular noun are:

• ova cesta — nominative
• ove ceste — genitive
• ovoj cesti — dative/locative
• ovu cestu — accusative
• ovom cestom — instrumental

Because cesta is changing form for the locative singular.

The basic form is:

• cesta = road

But after na in a location meaning, it becomes:

• na cesti = on the road

With the demonstrative:

• na ovoj cesti = on this road

So cesti is simply the locative form of cesta.

Ne smiješ is the 2nd person singular form of smjeti, meaning:

• you must not
• you are not allowed to

It expresses prohibition, not inability.

So:

• ne smiješ ≠ you cannot
• ne smiješ = you may not / you must not

This is very common in rules, instructions, and traffic language.

Because Croatian usually does not need subject pronouns when the verb ending already shows who the subject is.

Here:

• smiješ already means you are allowed / you may in the singular informal form

So ti is unnecessary unless you want emphasis:

• Ti ne smiješ pretjecati autobus. = You must not overtake the bus.
  This sounds more emphatic.

The normal neutral sentence simply leaves ti out.

Because after smjeti (to be allowed to / may), Croatian normally uses another verb in the infinitive.

So the structure is:

• smjeti + infinitive

Examples:

• Smiješ ući. = You may enter.
• Ne smiješ parkirati. = You must not park.
• Ne smiješ pretjecati autobus. = You must not overtake a bus / the bus.

This is the normal pattern.

This is about aspect.

Croatian verbs often come in pairs:

• pretjecati = imperfective
• preteći = perfective

In rules, prohibitions, habits, and general instructions, Croatian often prefers the imperfective verb, because it describes the action in a general sense:

• Ne smiješ pretjecati autobus. = You must not overtake a bus / the bus.

Using pretjecati sounds natural for a traffic rule or general prohibition.

Very roughly:

• pretjecati = the act/process of overtaking
• preteći = to overtake successfully, as a completed event

Autobus is the direct object, so it is in the accusative.
But for many masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: autobus
• accusative: autobus

That is why it looks unchanged.

Compare with an animate masculine noun:

• vidim čovjeka = I see the man

But with an inanimate masculine noun:

• vidim autobus = I see the bus

Croatian has no articles like the or a, so autobus can mean either depending on context.

In a sentence like this, English might translate it as:

• the bus
• a bus
• sometimes even buses in a rule-like context, depending on how the meaning is presented

Croatian leaves that distinction to context.

Yes, Croatian word order is fairly flexible, but some orders sound more natural than others.

The original:

• Na ovoj cesti ne smiješ pretjecati autobus.

This is very natural because it begins with the location/context.

Other possible orders include:

• Ne smiješ pretjecati autobus na ovoj cesti.
• Autobus ne smiješ pretjecati na ovoj cesti.

These are possible, but they shift the emphasis:

• starting with Na ovoj cesti emphasizes the road/rule location
• starting with Autobus emphasizes the object
• starting with Ne smiješ emphasizes the prohibition itself

So the original order is a very natural neutral choice.

Grammatically, yes: smiješ is 2nd person singular informal.

So it is directed at:

• one person
• in an informal or general you sense

This is common in learner examples and direct instructions.

If you wanted the plural or formal singular/plural form, you would use:

• Na ovoj cesti ne smijete pretjecati autobus.

That can mean:

• you all must not overtake a bus on this road
• or formal you must not overtake a bus on this road

Yes. In Croatian, ne is normally written separately from the verb:

• ne smiješ
• ne možeš
• ne znaš

So ne smiješ is just the standard way to negate the verb smiješ.

It is best learned as:

• smiješ = you may / are allowed
• ne smiješ = you may not / must not

Yes, very much so.

The combination of:

• a location first: Na ovoj cesti
• prohibition: ne smiješ
• action in infinitive: pretjecati

sounds exactly like the kind of language used for:

• traffic rules
• instructions
• warnings
• teaching examples about road signs

So even without extra context, it has a very regulatory or instructional feel.