I saw them installing them at Hamilton when I went into the city for a family event. I was amazed what percentage of the slow moving installers where just people who stood around sounding horns.
The FRA is quite serious about watchmen. This tidbit from CFR 214.329 may help:
Train approach warning shall be given in sufficient time to enable each roadway worker to move to and occupy a previously arranged place of safety not less than 15 seconds before a train moving at the maximum authorized speed on that track can pass the location of the roadway worker. The place of safety to be occupied upon the approach of a train may not be on a track, unless working limits are established on that track.
Track speed is 135mph in that are so that means is moving approximately 200 feet per second so they have to spread out and elongate the warning since the train would cover more than a half of a mile in 15 seconds.