Ignoring the rather stupid and irrelevant constitutional argument over rights to “privacy” and “separation of church and state” (the first was found to exist; the second is only a phrase used by Madison)…
A parking ticket cannot be compared to photo radar/stop-light tickets. First of all, you don’t receive a criminal summons with a parking ticket; instead you simply are advised of the fine levied for the fact the car was illegally parked. Second of all, since parking infractions are expressly civil in nature, usually the law establishes joint liability between the owner of the car and the person operating it illegally by parking it incorrectly. An example of the rules involved can be found in the Ohio Revised Code, Chapter 4521, Sections 1 to 10.
On the other hand, both speeding violations and red-light running violations DO usually include a criminal summons; show up at the court on such-and-such date at such-and-such time or all hell breaks loose. You can waive your right to contest the charge by the state by paying the pre-determined fine, in which case the state doesn’t require your presence to establish your guilt for the record.
Some jurisdictions have attempted to avoid these difficulties by modifying the statutes involved to change the nature of the infraction when photos are involved. Thus, a state might turn a photo radar/stop-light violation into a civil infraction, by enacting a statute different from the basic traffic law, or by limiting the punishment of the violation to a civil procedure similar to parking tickets. However, if the law of which the owner is accused of violating still declares that “No person shall drive faster than …” or some similar such wording, then the attempt to turn the violation into a civil infraction still fails to provide due process, for the owner hasn’t committed a violation of the law. In addition, there is a difference between a local authority enacting an ordinance that says, “Don’t park here without paying $1.00 per two hours,” and the state saying, “Don’t go faster than 35 on a state route in a municipal corporation’s boundaries.” Still, at least such an attempt, if it decriminalizes the offence, has reduced the constitutional infirmity.
This discussion applies as well to jurisdictions where the entire moving violation process has been decriminalized. Of course, in such states, you can’t be arrested for moving violations, and you never have to appear anywhere if you fail to pay. Presumably, in such states, the traffic laws have been modified to replace language saying “no person shall” with language saying “it is a violation to …”
One side note: while the police might want to question the owner of the gun used in the murder, the would have trouble arresting the owner of the gun absent evidence that the owner was the shooter.
For what it is worth, the Sixth Amendment claim is probably a red herring. You are never “accused” by the machine; there would always be the ability to establish the accuracy of the machine through witnesses. The effectiveness of this evidence might at times indicate a need to return a not guilty verdict, but there is no constitutional infirmity unless there can be no witness.
Oh, and Sam Stone, a citation to the study you assert would be helpful. Amazingly enough, jurisdictions are much less interested in revenue than safety when it comes to things like red lights and speed limits; usually because some disgruntled citizenry is trying to make their life miserable. I doubt seriously the claim your street in question didn’t have the new limits posted; failure to post the limits would leave the speed limit at the “prima facie” speed for the type of street involved, and a ticket based on a lower limit would be dismissable accordingly (probably true in Canada as well as the U.S.). Unfortunately, we tend not to notice such changes driving, as I found out on my first ever driving test for a license, when I ran a newly installed stop sign at an intersection through which I had biked numerous times. I gathered from the comments made by the examiner I wasn’t the only one who had failed to notice the new red octagon. 