Cars can maneuver much more readily, travel at much lower velocities, have road traction so aren't affected by their gravitational attraction to each other, and

Cars can maneuver much more readily, travel at much lower velocities, have road traction so aren't affected by their gravitational attraction to each other, and don't have a third axis of separation (altitude) to deal with, including changes in that axis due to micro-gravity and atmospheric fluctuations. This makes long-term modelling of orbits/intersections very difficult, and the error bounds are much too large to give any reliable projections on the order of 5 metres. We would know that they came within 5 metres of each other when they come within 5 metres of each other; not long before.