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Depends on how one reads the 4th Amendment. If probable cause and a warrant are required to carry out unreasonable searches, then a perfectly robust interpretation is that they are not required to carry out reasonable ones.
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The applicability or otherwise of the Amendment therefore hinges on what is or is not unreasonable, and not on the search itself.
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The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated
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and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized
If you read these two things separately, then it would appear clear that you are protected from unreasonable search, and that to obtain a search warrant (which IMHO implies a reasonable search if a warrant is issued) there must be probably cause and evidence to support the warrant.
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Plus, at least in the case of intoxication or seatbelts, most of the symptoms of legal infractions are in plain view - you can see whether or not a driver is wearing a seatbelt through the windows, and you can reasonably suspect intoxication from the way they drive. Anyone daft enough not to buckle up or to drive drunk deserves a little mild rights infringement, IMHO.
These infractions are in plain view. However, restricting the flow of traffic and stopping vehicles to look inside is clearly intended to improve law enforcements ability to identify these problems, at the expense of all the law abiding citizens that are not violating the law. They examine every vehicle, therefore it is not a specific search.
Look at it from another perspective, using your argument. Would it be OK for law enforcement to require an unobstructed view through your homes windows so that they could walk up to your windows and peer inside to determine if anything illegal is going on? That is essentially what they are doing to motorists.
What if they started coming to everyones house demanding you produce ID and a deed or mortgage paper for the property? They do this to motorist all the time with random safety stops and the like.
EDITED TO ADD:
I think Ben Franklin said it best:
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"A man who will give up liberty for security deserves neither liberty nor security."