This blogpost is exclusively written for structural engineers. Non-engineers may find this blogpost unentertaining.
The analyis and design of post for guardrail are quite simple. Being a flexural member, a guardrail post is normally idealized as vertically oriented cantilever beam -- fixed at the bottom end and subjected to a concentrated load at the top free end.
The concentrated load at the top of rail post originates from a single 200-lb concentrated load and a 50-lb/ft uniform load specified for railing system by some building codes. The two loads, however, are not applied concurrently.
When the 200-lb concentrated load, P, is applied at the top of rail post, portion of the load is distributed to the adjacent posts on both sides thru the horizontal rails -- thereby reducing the effective load on the rail post. This reduction is dependent on the stiffness of posts and the stiffness of horizontal rails that comprised the guardrail system. Generally, this type of loading does not directly define the rail post spacing.
When the 50-lb/ft uniform load, w, is applied at the top rail, the amount of load that a rail post sustains is dependent on the spacing between posts -- which translates into load tributary length. The wider the post spacing, the greater the load tributary length will be and, subsequently, the greater the load that a rail post sustains. Hence, this is the type of loading that determines and defines the post spacing. The maximum spacing that rail post may attain is a function of its strength. Although the top rail may also control the spacing of rail post, this discussion only covers the rail post. Let us just assume here that the top rail is not the controlling guardrail member.