[dingaling]

The OP means that satnav ensures that two aircraft on a reciprocal or identical track will now pass over one another, whereas there used to be navigational slop that would give some unintentional but useful lateral displacement.

[cjrp]

Ironic that you should use the term 'slop', since that's exactly how they avoid flying the same route across the Atlantic tracks for example (Strategic Lateral Offset Procedure).

[phumbe]

Also worth noting for the transatlantic paths is the direction of traffic. When it's morning in North America, traffic is predominately westbound. When it's night, traffic is predominately eastbound.

Right now, it's the middle of the day (12:50p Central). There's a bit of a mix[1], but it's still predominately westbound.

The consequence of this is that most traffic will be separated by at least 2000 feet. Eastbound traffic will (generally) be at odd Flight Levels (so 29000 feet, 31000 feet, etc.) while Westbound traffic will be at even FLs (30000 feet, etc)[2]. This assumes RVSM airspace.

[1] I'm just looking at the transatlantic traffic on https://flightaware.com/live/

[2] https://www.law.cornell.edu/cfr/text/14/91.179