Across, not down
CAT make fine Cable Avoidance Tools. Available in a range of bucket sizes with teeth to yoink up a cable so you know to avoid it in future.
Well mapped can also be a matter of interpretation, or just availability. So one being how well any survey data gets logged, the other knowing how to find any online portal or contact. Sometimes GIS doesn't give much accuracy beyond a rough idea which side of a road infrastructure might be on. I've also used GIS that have Google maps as a front-end, and although the survey data is accurate, Google's mapping data isn't.
But civils contractors should also use lo-tech approaches, ie walking the planned excavation and looking for suspicious lines of tarmac, or duct covers. But that can be fun, ie I have Torch chambers near me, and can't remember who their fibre ended up with. Then there's the proper methods, like using detectors & spray chalk to indicated buried stuff, and then care when digging. Fibre routes should have metallic locator wires so they can be found, and plastic tape above the route warning of buried cables.
But that doesn't always stop a merry digger. VM (and other new entrants) also have a challenge given utilities were/are assigned layers or burial depths to deconflict utilities. Telco being newest, it's usually shallowest so it takes the hit first.