There's a lot of detail here: theforce.net
At the time of the Great Sith War hyperspace travellers relied on a network of hyperspace jump beacons stationed at the nodes of safe routes. These acted as milestones of the galaxy and contained computers to provide safe jump coordinates to departing vessels. In that age most vessels relied on these fixed facilities for jump calculations, rather than supporting the complex onboard navicomputers which were in fashion during the Palpatine Era. The computers aboard the jump beacons were able to compensate for local hazards and galactic drift in order to provide safe passage to a number of neighbouring beacons and systems. (For the sake of stability, most beacons are located in interstellar space, outside the gravitational wells of nearby star systems.) This system reduced travellers' independence of movement but improved jump reliability, provided that the beacon was in working condition. Only the specially-equipped scout ships of explorers were equipped with navicomputers and capable of jumping into unknown territory. At any given time almost one fifth of beacons might suffer from some malfunction, despite the continual repair efforts of the Republic Spacelane Bureau jump beacon patrols.
Exit from hyperspace is automatically triggered by the navicomputer or equivalent systems when the ship has travelled in the correct direction for sufficient duration. Reentry is not directly connected with the proximity of any massive celestial body at the destination point (because it remains possible to jump back to realspace in the middle of empty interstellar space); nor with any other external environmental factor. Safety systems, however, may trigger automatic drop back to realspace if sensors have enough time to detect the ship's entry into the margins of a gravity well. (The observed circumstances of the Millennium Falcon's arrival at Alderaan indicate that a field of decimetre-sized rocks is not sufficiently concentrated to trigger early cut-out.) User-friendly starship navigation and hyperdrive systems will issue an audible or visible alarm just prior to reentry, and prudent crew will then raise their shields and power up their sublight engines.
Regardless of the methodology used, it seems there's a certain amount of risk to hyperdrive technology with or without routes. As to mapping new routes quickly and safely, I'm not sure there's a way. But I'm not a hyperdrive engineer. Others here may be able to provide more information.