In a Sherlock Holmes story, the protagonist uses forensic evidence but he thinks about what the evidence means.

The police, on the other hand, want a magic oracle device that spits out names and current addresses, and they treat forensics like it was such.

In the United States, having a valid explanation for why your fingerprints were on some object wouldn't necessarily get you freed, it wouldn't necessarily prevent prosecution. Some asshole DAs here would take that to trial (or extort you into a plea bargain, at least).

someone to get arrested so it looks like they are doing a good job so the chief gets off their back, the chief wants this because re-election is coming soon and doesn't want to look weak on crime. Neither one of them can go pick up some random black guy for the crime any longer without too much media attention so 'forensic' evidence is what they try to use today.

Fortunately, I had a tail light out -- not a brake light, just the ones that idle on when the car is running. When he asked "do you know why I pulled you over?" I said "Because I have this tail light out: but the brake lights are fine, I have this appointment to fix them tomorrow, and here is this warning that this other officer gave me about the problem, which clearly says that it has to be fixed by the end of the week."

He said out-right "no, that's not why I pulled you over, you ran that stop sign," and I said, "no, I came to a complete stop, I always do, it's one of those disciplines I have that really annoys my girlfriend..." and such. Probably I shouldn't have even tried to argue it? But I did.

In the end, he came back with a ticket for "driving a defective vehicle" -- because the tail light was out, and despite the fact that he should have ideally deferred to the earlier warning. I asked him about it and he said, "you didn't fix it fast enough!" brusquely. Probably he wanted to lean on my immediate statement of "this is broken but I am fixing it tomorrow" in any trial, like, "he immediately confessed to the crime."

This turn of events was quite fortunate as when the county prosecutor saw both pieces of paper side-by-side (the warning, "fix by the 9th", and the ticket issued on the 7th, or whatever), she agreed to drop the case before it went to trial. (I had also been told by a police clerk that the judge was pretty lenient about traffic violations, so that may have also been in her mind.)

Apropos of anything else - you DID cut him off, and you did fail to yield (Stop signs are also Yields). His argument about you running the stop sign is invalid, sure, if you were at a complete stop, but if you're forcing someone who did not have to yield to, by your own words, 'slow way down' to match you pulling out in front of them, then you were in the wrong.

I don't think I 'failed to yield' to him because I exited the intersection before he entered it: he only really had to slow down once he got into the road I was in. But had he given me a ticket for that, I might have just taken responsibility and paid the fine. I mean, I think I could plausibly argue the case in court because I "made it" well ahead of him. But then again, he may have started slowing down much sooner, and it could be a thorny argument.

What interested me most about it at the time was: it was a 3-way T-shaped intersection formed by taking a 4-way intersection and blocking off the North end with construction work. So the North-South road had no signs, but the general rule for such intersections (and I think it's explicitly written down in most states, but I'm not sure) would be that the South traffic has to yield to East/West traffic: in other words, in combination with the East/West stop signs, I think the law-books would imply that the road should have implicitly become an all-way stop.

That's precisely why he asked the question in the first place. Never admit to anything.

A few years ago, I got a ticket for driving with an expired inspection sticker. I knew it was expired and it was my fault so I wasn't going to try to argue my way out of it but when the officer asked if I knew why he pulled me over, I said "I have a pretty good idea, officer but I'm not going to say it out loud."

  Prosecutor: In Officer Wiggum's evidence he stated that you said you had a
  "pretty good idea" why he pulled you over.  Is that correct?
  You: I guess.
  Prosecutor: And what was that idea?

All of that is moot. Where I live, there's no prosecutor involved for traffic tickets.

I think we all want tools that help us do our jobs quickly and more reliably. Forensic science is, in aggregate, a tool for discovering what's true. Improvements in forensic science are, on balance, a net positive.

In the United States, nothing prevents prosecution. The state can bring whatever charges it wants. The bulwarks of our freedom are in the courts, the ballot box, and in the enumerated Bill of Rights.

It's the jury of our peers and a speedy trial that check the ability of prosecutors to do us individual harm. As a society, it's our power at the ballot box that determines who we admit into the job of prosecution.

I can't help but feel there's something vital missing from police, DA, etc training. I'm reminded very distinctly of my ethics class (which was required for graduation), where we were told the horror story of the THERAC-25. Even now, when I work on software for the military, I am very conscious of what my decisions may cost. Why aren't police, judges, district attorneys and forensic scientists required to take classes that tell them of stories like this one? I feel that juries aren't enough, especially when they are surrounded by those with much more experience and comfort in the courtroom.

This could be selection bias at work. Perhaps you terminate conversations with less intelligent officers earlier, without realising the reason you do so. So, these less intelligent ones are forgotten, or removed from your sample because the conversation was not long enough to find strong evidence about intelligence.

No. He's just poor at judging intelligence, like everyone else.

If they are polite and don't raise their voice and there are no obvious deficiencies in their vocabulary or pronunciation, and they seem to be minimally informed on current events and trends, you couldn't tell an idiot from a genius. You have no ability to see their thought processes, and you've never really bothered to try to infer them.

Studies even suggest you don't know what's going on in your own brain either. This despite the fact that you are actually privy to at least some of your own thought process.

[citation needed]

The citations will probably be helpful for you to find the studies and similar ones.

My favorites:

Why did we think we dreamed in black and white?

http://www.faculty.ucr.edu/~eschwitz/SchwitzAbs/DreamB&W.htm

How well do we know our own conscious experience? The case of human echolocation.

http://www.faculty.ucr.edu/~eschwitz/SchwitzAbs/Echo.htm

edit: here's an interview - http://www.3ammagazine.com/3am/the-splintered-skeptic/

So yeh. There are no limits and no meaningful rules as to what they can and will do. Certainly many feel no moral or ethical constraints on their actions.

http://www.law.northwestern.edu/legalclinic/wrongfulconvicti... http://www.pbs.org/wgbh/pages/frontline/shows/case/etc/updat...

You mentioned that you felt lucky; sad to say, I think you're probably right. What a nightmare of an experience.

Never talk to police without a lawyer is good advice, but that advice literally translates to "Never talk to police without paying someone $300 per hour to do it." Meanwhile, police, at no additional cost to themselves, can essentially compel you to talk to them any time they want. So it's pretty easy to push people around with that dynamic, yet most people I talk to seem to bury their head in the sand on the issue.

I'm reminded of:

George Aaronow: When I talk to the police I get nervous.

Ricky Roma: Yes. You know who doesn't?

George Aaronow: Who?

Ricky Roma: Thieves.

So I suspect that finding one good print and extrapolating a suspect from it is probably pretty common. Flawed obviously, but common.

That's why such evidence is supposed to be supportive of a larger case. Of course there will be a fingerprint on almost any given item. If you have a complete biometric database of every person in the country you will be able to match it, but that doesn't mean anything at all. If the person matched happens to be local it still doesn't mean anything by itself. In this case there was a plausible innocent explanation of how it got there, but in most cases people won't know how they spread their prints or DNA.

This is exactly how the legal system in the UK works - you arrest people at a relatively low burden of proof, and that gives you the power to detain them for a few hours while you question them, search for evidence, etc. You can't do any of that if you go over and "ask"