The runners will natch encounter more police presence as they move up the food chain. Robbing a snack shack will be a lot easier than breaking into a bank vault, and that goes for every level. More valuable things tend to be surrounded by more cops, and on top of that, there will be more cameras to snap their photos and more cooperative witnesses to give the fuzz information as they attack more valued targets. If your getaway van has been seen during the course of a run, then the cops will notice it while they're on patrol, and they'll be more likely to pull you over based on whatever probable cause they can pull out of their ass. This should carry over to your campaign, too. If they're pulling off runs in areas that aren't friendly to the cops, they'll quickly become big fish in a small pond, so to speak. In that case, offer them jobs that put them at more risk. If they're already moving through areas with a big police presence, turn up the heat on them. What if the cops have finally cracked open a lead on the job the runners finished two sessions ago? All of a sudden, they'll remember that they have to cover their tracks or else they won't have a chance of making it to the Italian restaurant that Mr. Johnson's meeting them at.
