Robert and Mike, if it's any consolation, I agree somewhat with both of you. Let me qualify this by saying I've been an inspector (civil and structural) on a number of public works projects (including bridges, river channels and dams), and I think failures like this can be the result of bad ethics, bad design, bad judgement, or any combination thereof.
Sure, there are inspectors (or other officials) on the take. In my experience, they were very rare. (I never saw or knew of any in the part of the country I was working in.) Then there are inspectors who don't really care, and are only there to collect the paycheck. (Getting paid to watch other people work...seems like easy money. I'm guessing that may be why the guy up the road from Robert is sitting a mile away from the site in his pickup, assuming he wasn't doing something legit like checking grade stakes.) Also keep in mind that sometimes inspectors are only required to do periodic spot checks, not necessarily watch every shovelfull of dirt turned. On a number of occasions I was working on three or four different sites daily. Or in other cases, the site itself might be miles long. Needless to say the inspectors don't necessarily see it all. To categorize all inspectors as dishonest or lazy is an insult to all the dishonest and lazy people out there.
Seriously, it's like saying everybody with a cell phone is a drug dealer.
Then there are designs that are not sufficient. Engineers build in a "safety factor", but sometimes new construction methods are not fully proven, and the expected performance doesn't happen. Or technology advances and we learn what we thought was plenty good really isn't. For example, in the past decade or two, structural engineers here in California have learned that some of their design guidelines for earthquake loads was off, and as a result the building codes are more strict now than they were 30 or 40 years ago. And of course there's always the underlying issue of trying to do it with as little money as possible.
There are also daily judgement calls that get made regarding the interepretation or execution of the specs. Not only on the inspector's part, but on the part of the engineers and contractors, too. (Here again, sometimes the cost plays a role.) I have yet to see a project that matched the specifications exactly. There are times when, in the process of construction, it's discovered that the prescribed method or material is not feasible, or a less expensive alternative is proposed. Any substitutes are generally intended to meet or exceed the specs, but as I said, sometimes judgement calls are made. And sometimes they have to happen fast, and things can and do get missed.
All that said, I'm not claiming any of these are the reason for the bridge collapse, but those are my observations as a former inspector.
And my thoughts and best wishes go out to the victims and those close to them.