Clear zone and barrier length of need on the PE Civil Transportation exam

A roadside problem on the Transportation exam usually asks one of two things: how far from the road must this obstacle sit, and, if it can't move, how long must the barrier in front of it be? Both are worked from the AASHTO Roadside Design Guide (4th ed., 2011), supplied with the exam. The Handbook doesn't cover either. The Guide treats clear zones in its roadside-topography chapter, and barriers and length of need in its roadside-barrier chapter.

What you actually need to own

• Reading the clear-zone table. Enter it with the design speed, the design ADT band and the slope, which picks a foreslope or backslope column. Each cell is a range, so check which end the problem asks for. The distance runs from the edge of the through traveled way; shoulders, and auxiliary lanes that don't act as through lanes (a turn lane at an intersection), sit inside it.
• Slope classes. Slopes of 1V:4H or flatter are recoverable and count. A slope between 1V:4H and 1V:3H is traversable but non-recoverable: it doesn't count and the clear zone can't end on it. The part of the clear zone that would fall on it becomes a clear runout area beyond the toe, at least 10 ft where practicable, and the table is read for the steepest recoverable foreslope before or after it. A slope steeper than 1V:3H is critical and earns no runout credit. If it begins inside the clear zone, flatten or shield it.
• Cut sections. With a preferred ditch the backslope is more recovery area; the larger of the foreslope and backslope values governs.
• Curves. CZ_c = L_C × K_cz, with the factor read by radius and design speed. It applies on the outside of the curve only.
• Barrier choice. The barrier's dynamic deflection must fit between its traffic face and the face of the object it shields. The flare rate depends on design speed, on whether the barrier is inside or beyond the shy line (measured from the edge of the traveled way), and, beyond it, on whether the system is rigid or semi-rigid.
• Length of need. L_A runs from the edge of the traveled way to the far side of the hazard; L_R is the runout length, read by design speed and traffic volume; L1 is the tangent barrier run upstream of the hazard and L2 the barrier offset. With flare b/a, X = [L_A + (b/a)L1 − L2] / [(b/a) + L_A/L_R]; with no flare, X = (L_A − L2)/(L_A/L_R). X is where the barrier line meets the runout line from the hazard's far edge to a point L_R upstream. L_A stops at the clear-zone edge when the hazard extends past it. On the outside of a curve with flat, traversable ground, a vehicle leaves on a tangent, and that tangential path governs when it's shorter than L_R.

Where people lose points

• Posted speed instead of design speed, which can cost you twice in one problem: in the clear-zone table and again in the runout-length table.
• Wrong band or column. Check which ADT band the volume falls in and which column contains the drawn slope.
• Measuring from the shoulder edge. Clear zone, shy line and L_A all start at the edge of the traveled way. Deflection space runs from the barrier face to the object face.
• Mishandling the runout area. Stopping at the toe of the non-recoverable slope is too short; moving the whole clear zone past the toe is too long. Only the part that fell on the slope moves.
• Curve factor on the inside. The side toward the center keeps the tangent value.
• L_A to the far side of a wide hazard. For a pond or slope running past the clear zone, L_A is the clear-zone width.
• The straight-road equation on the outside of a curve, where the tangential path can be shorter.

How to study it

Take one obstacle beside one road. Find the tangent clear zone, then move it to the outside of a curve, then put a non-recoverable slope in front of it, noting what changes. Then shield it: read L_R at the design speed, compute X with and without a flare, and check that the two lines really meet at X. The curve is the one from horizontal curve elements, and the shoulder breaks come from superelevation and runoff.

Part of the Roadside and Cross-Section Design area of the PE Civil Transportation exam. → Start practicing free — the whole practice bank, free with an account.