4.1 Forgiving Roadside and Clear Zone Design

Forgiving Roadside and Clear Zone Design

The forgiving roadside concept is a fundamental tenet of modern transportation engineering, designed to reduce run-off-road (ROR) crash severity. Pioneered by AASHTO in the Roadside Design Guide (RDG), this philosophy recognizes that drivers occasionally leave the roadway due to fatigue, distraction, or emergencies. A forgiving roadside provides errant vehicles a reasonable opportunity to recover control, stop safely, or traverse the area without colliding with a rigid object or overturning.

The Clear Zone Concept

The clear zone ($CZ$) is defined as the unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles. The clear zone width includes shoulders, auxiliary lanes (turning or climbing lanes), and traversable slopes. It excludes standard bike lanes or sidewalks unless they are traversable and free of hazards.

The required width of the clear zone is not a single fixed value; rather, it is a function of three primary parameters:

1. Design Speed: Higher design speeds require wider clear zones because errant vehicles travel further off the road before stopping.
2. Design Volume (Average Daily Traffic, ADT): Higher-volume roadways have a higher statistical probability of run-off-road events, justifying a wider clear zone.
3. Roadside Slopes (Foreslopes and Backslopes): Steeper slopes accelerate or destabilize vehicles, requiring adjustments to the clear zone width.

For example, a low-volume local road with an ADT under 400 and a design speed of 40 mph might require a clear zone of only 7 to 10 feet. Conversely, a high-speed freeway with a design speed of 70 mph and an ADT over 6,000 may require a clear zone of 30 to 34 feet or more.

Horizontal Curve Adjustments

Vehicles are more likely to run off the road on the outside of horizontal curves. Thus, the baseline clear zone must be adjusted on the outside of curves using a correction factor ($K_{cz}$):

$CZ_c = CZ \times K_{cz}$

The factor $K_{cz}$ ranges from 1.1 to 1.5, representing a 10% to 50% increase in clear zone width. For example, at 60 mph with a 1,500-foot radius, $K_{cz}$ is 1.2. If the baseline clear zone is 20 feet, the corrected clear zone is 24 feet.

---

Slope Design and Vehicle Stability

Roadside slopes are classified by traversability and a driver's ability to regain control. AASHTO categorizes slopes into three types:

Clear Zone on Non-recoverable Slopes

Since a vehicle will likely traverse the entire width of a non-recoverable slope, the slope width cannot count toward the clear zone. Instead, the clear zone must extend beyond the toe:

$\text{Total Required Clear Zone} = \text{Width of Shoulder} + \text{Width of Non-recoverable Slope} + \text{Clear Zone Recovery Area at Toe}$

The recovery area at the toe of the slope must be flat (1V:6H or flatter) and must be at least as wide as the remaining clear zone width that was not satisfied before the slope.

Roadside Ditch Design

Roadside ditch design within the clear zone must also prevent vehicle rollover. V-ditches present high risk as they abruptly change a vehicle's roll angle. AASHTO recommends using round-bottom or trapezoidal ditches with flat bottoms (4 to 8 feet wide) and flat slopes (1V:4H or flatter). If a V-ditch is used, slopes must be flatter (1V:6H) to prevent wheels from catching. The transition between the shoulder and ditch should be rounded.

---

Roadside Obstruction Handling

Any rigid, non-breakaway object within the clear zone is classified as a roadside hazard. Typical hazards include trees with a diameter greater than 4 inches, utility poles, drainage headwalls, bridge piers, and steep critical slopes.

AASHTO defines a strict, non-negotiable hierarchy for treating roadside obstructions. Designers must address hazards in the following order of preference:

1. Remove: Eliminate the hazard entirely (e.g., cut down a large tree, remove an unused structure).
2. Redesign: Modify the hazard to make it traversable (e.g., flatten a steep ditch slope, convert a steep headwall into a traversable grate).
3. Relocate: Move the hazard outside of the clear zone (e.g., move utility poles further back from the travel lane).
4. Reduce (Breakaway): Modify the hazard to yield or break away upon impact, reducing deceleration forces (e.g., use breakaway bases for sign supports and light poles).
5. Shield: Install an appropriate barrier system (e.g., W-beam guardrail, concrete barrier) or crash cushion to shield the hazard. This is only done if the barrier itself is less hazardous than the obstacle.
6. Delineate: If no other option is economically or physically feasible, install warning signs or reflectors. Delineation does not reduce crash severity, only visibility.

By systematically applying this hierarchy, engineers minimize the need for guardrails, which are themselves roadside obstacles that present a collision risk.