I don't know when they started wearing the vests.

but m.o.w. workers on the Santa Fe and later BNSF all wore the high visibility orange vests and hard hats from at least the 1990's on.  The hard hats in the BNSF era are all orange with the green "circle-cross" logo and BNSF stenciled on the hats.  Depending on when the railroad workers started wearing the orange vests and hard hats, the highway workers might not be far off.

1970's

Having hired on in 1970 I can guarantee they weren't wearing vests.

I would look for "ranchers" (not cowboys) or generic people in pants and shirts.  In the 1970's hard hats would be becoming common on MoW.  Mechanical forces would be wearing denim overalls as would section gangs.  Train crews would be mostly jeans or work pants and shirts.

Possible source

You might find some suitable that are already painted and there are also unpainted offerings from People Scale. I have bought a large number in N scale and am painting them and found them to be fair quality. Here is a link for the HO scale painted figures:

http://peoplescale.com/Painted-Figures-HO-Scale-187_c192.htm

People Scale is based in Carlsbad, CA, and ship quite quickly. You may find other items they have to your liking as well.

Charlie recently wrote an article in MRH on painting figures.

more 1970s

ONR carmen and shops crew in the late '70s wore blue hard hats (except for the guy who fell into the turntable pit and, not being injured, was awarded - and wore - a gold hard hat.  Supervisors skulked about in white hard hats.

So you could have people in overalls and people in suits both with hard hats.

David

You can use unpainted figures from Atlas,

Model power, or Kibri. Don't forget that in the seventies, lots of railroaders were wearing those funky colored engineer hats, and some older guys still had bib overalls, for they were easier to wear because they held your pants up instead of a belt. So a mix of styles will work. I model '68 and that's what i use.mh