I've been a bit slack answering email questions. Here we go;

Why not use prisoners, riff-raff & layabout scum to pick up litter, wash marker pegs ?

In a word safety.
You have to work within signs put up by someone qualified to do so, have a Site Safe Passport, vehicles with flashing lights etc. etc. While running down neer-do-wells in your car may be fine sport, the outcome would keep a big bunch of
bureaucrats busy for months.

What do you find on the road, anything valuable?

You name it, we find it.
We often find tools, stuff that has bounced off trailers and trucks, spare wheels, things that have been heaved out windows after domestics (CD's figure prominently), and zillions of wheel trims & hub caps.
You can also find dead dogs & assorted wildlife, the odd body, ( not us, fortunately) stolen cars, furniture, car parts and so on.
Alas no jewels, or gold bars as yet...

What are the strange numbers on bridges ?

In N.Z. Transit use a simple bridge number, then a route position, so Bridge No. 1234 then the route position (RP) which is really just how far from the beginning of the highway it is...
"The longest bridge on the state highway network is the Rakaia River bridge on SH 1S at RP 381/18."
= State Highway 1 South, 381+ 18 kilometres from Picton

What do you think causes the most crashes?

On our stretch of highway, speed & fatigue by a country mile. Usually a combination of both. The driving on auto pilot syndrome.
"Driver fatigue is difficult to identify or recognise as contributing to a crash. This means it’s likely that fatigue is under-recorded, and contributes to more crashes than we realise. Australian estimates indicate that fatigue accounts for up to 30 percent of single-vehicle crashes." LTSA