Space is big. So big that, for the most part, it's really unlikely to accidentally run across anything. At all. Nevertheless, there are varying levels of probability of an encounter in space in the known galaxies, most of which relates to issues of proximity and detectability. Let's start off by describing the basic differences in the types of locales for ships.
Starting small, let's consider the atmosphere around a planet or moon. By extrapolation, this can also apply to all fluid layers of water planets and gas giants. The chances of bumping into something while atmospheric is directly proportional to the population of the planet. Barren lifeless worlds are going to have pretty empty skies, while bustling city planets may well have lines of traffic that start in low orbit.
Detectability while atmospheric depends primarily on speed with respect to the density of the atmosphere. Obviously, it is impossible to travel superluminally or relativistically for longer than a bright and fatal flash when atmospheric. For most habitable planets, it is impossible to travel more than a kilometre per second without creating extremely noticeable shock waves and contrails. However, if a ship is moving slowly or stationary with respect to the planet's surface, they are extremely difficult to locate other than by being extremely close to them. Etheric sensors will be utterly drowned by the planetary mass, and detection by electromagnetic sensors will be heavily influenced by the opacity of intermediately-located matter. Like, say, hiding in the clouds, or amongst some concealing terrain. Still, if it's a crowded city planet, there may well be eyes throughout the volume of the atmosphere, thus trumping the detectability equation by virtue of ubiquitous proximity.
The types of vessels that might be encountered in an atmosphere starts at skateboards and has a maximum size limited primarily by the realistic interaction of the ship and the terrain.
Orbital space primarily refers to the volume of space noticeably affected by a planet's gravity well. As with atmospheric travel, the probability for encountering something in orbit is largely related to the population of a planet by space-faring beings. For unpopulated planets, there is only likely to be some objects in natural orbits, while the orbits of city planets are usually the most dangerous region to fly through due to rushing traffic.
There should be some distinction made between natural and artificial orbits. Objects assuming a natural orbit will travel in a predictable conic-section path depending on vector and planetary mass, with all stable orbits being essentially elliptical or circular. Artificial orbits require propulsive power, and can describe any sort of movement.
While it is technically possible to achieve superluminal or relativistic travel without automatic death in nominally orbital space, it's akin to getting a hole-in-one in golf. The odds of it going awry are pretty good due to the curvature of space-time and the presence of pesky matter. Other than that, most ships are capable of full-acceleration manoeuvres in sub-luminal.
The types of vessels that can be encountered in orbit is limited only by the ability to employ sufficient thrust to climb out of the gravity well. For orbits that have space elevators, this region may also include bicycles.
The patterns of travel in interplanetary space are generally in ever-shifting trails between the planetary bodies and other significant locations in the system, and out to the nearest hyperspace-lanes. These trails are usually a bit loosely-knit, in order to give ample clearance between craft of significantly different velocities.
Superluminal travel is not terribly difficult in interplanetary space, and it turns what could be a trip of hours even for high acceleration craft into the blink of an eye. However, this is usually done by first travelling subluminally out of the disc of the star system - for the dual purposes of avoiding the bulk of the stray matter in the system and for getting out of the reach of anyone that might want to take a pot-shot at a superluminal vessel. The ramifications of this will be discussed more in the SUPERLUMINAL TRAVEL section.
Only vessels capable of significant delta-V are likely to be encountered in interplanetary space. This generally implies a lower limit of "hopper class" vessels - so no bicycles.
The majority of the void between stars is considered interstellar space. While there are vessels in transit through interstellar space at subluminal speeds, they're essentially considered stationary waypoints for the purposes of most AIF travel. In this classification of space, superluminal is the only travel mode that matters. Like interplanetary space, most vessels tend to keep to narrow space lanes, only in interstellar space this is far more extreme. In addition to the natural mathematic superposition connecting destinations, there is also an element of safety and discretion in travelling along space lanes. Being much off of a space lane is often like being in the middle of nowhere.
Obviously, this locale also largely relies on the SUPERLUMINAL TRAVEL section.
The vessels encountered in interstellar space will generally reflect a balance of size and speed commensurate with the distances involved. As the typical speed capable by contemporary superluminal craft is 0.1 to 2 Parsecs per hour, and the typical distance between populated star systems is about 10 parsecs, slower vehicles are usually equipped with facilities for occupants lasting at least 100 hours, while faster vehicles only require facilities for tolerating 5-hour jaunts. In practice this tends to weed out smaller hopper-type craft (other than in desperate situations) and traffic is primarily shuttles and freighters.
Intergalactic space is much like interstellar space, except that there are no space lanes. This is primarily because even with superluminal travel it generally takes centuries to cross intergalactic distances, so virtually no ships venture there. The only things encountered in intergalactic space are deposited by a fold space generator, or old.