Playground slides are somewhat unique because it's a stationary apparatus that uses gravity, with a change in elevation of a smooth surface, which enables a child to gather speed and momentum as they progress from the top to the bottom. Older slides were really high in elevation, often made of metal (that got quite hot in direct sunlight), had rather steep drop angles to generate greater speed, lacked sides high enough to avoid falling over the edge, sometimes side-by-side to encourage racing (slide racers), and one could use it in almost any position as they flew downward on it... no matter how unsafe it might be. 

Perhaps the biggest mistake parent make when selecting a playset for their residential back yard is choosing any playset that has a slide with any bump in it for a toddler or preschooler who doesn't weight much yet. This bumps can send a young child airborne when they hit the bump, landing backward on the neck and head when they land back on the slide, flipping over the side, or falling forward into the bottom of the slide on their face. None of those are good options, so it's best to avoid those types of slides for very young ones. The elevation of the overall play equipment should be less than 4 feet in maximum height. If the children using a playground are ages 6 to 12, then the maximum height of any playset should be 8 feet (including the highest point on the entire playset, even if this is the roof). 

The safety zone off the end or landing zone of any slide should be extended beyond the minimum 6 foot measurement, to allow for the forward momentum and area where impact is most likely. Taking the change in elevation measured between the highest and lowest point on a slide where a child would be seated, then adding this to 6 feet, is a good place to start for a more appropriate safety zone. With greater angles and speed, more distance for the safety zone may be warranted.