Clear Zone Concepts
A clear zone is the unobstructed, traversable area provided beyond the edge of the traveled way for the recovery of errant vehicles. The design of the clear zone is a primary safety concept in roadway design, focusing on forgiving roadsides. The primary reference for clear zone design is the AASHTO Roadside Design Guide (RDG), Chapter 3.
Clear Zone Determinants
The width of the clear zone is not a single fixed number. It is determined dynamically based on three main parameters:
- Design Speed: Higher design speeds require wider clear zones because vehicles traveling faster will run off the road further.
- Traffic Volume (ADT): Roadways with higher Average Daily Traffic (ADT) have a higher probability of run-off-road events, justifying wider clear zones.
- Roadside Slopes: The slope of the roadside (foreslopes and backslopes) affects how easily a driver can regain control of a vehicle or how far the vehicle will travel down a slope.
Clear Zone Table Lookup (RDG Table 3.1)
On the PE exam, you will typically locate the baseline clear zone width using a table. You must match the:
- Design speed range (e.g., $\le 40\text{ mph}$, $45\text{-}50\text{ mph}$, $55\text{ mph}$, etc.)
- ADT range (e.g., $< 750$, $750\text{-}1500$, $1500\text{-}6000$, $> 6000$)
- Roadside slope ratio (e.g., 1:6 or flatter, 1:5 to 1:4, 1:3)
Classification of Roadside Slopes
Roadside slopes are categorized based on their traversability and the likelihood of a vehicle overturning:
| Slope Ratio | Type | Driver Control & Recovery | Safety Implications |
|---|---|---|---|
| 1:4 or flatter | Recoverable | Drivers can generally retrieve control of their vehicles and steer back onto the roadway. | Clear zone is measured continuously across these slopes. |
| 1:3 to 1:4 | Non-Recoverable | Drivers cannot easily retrieve control or stop, and the vehicle will continue to the bottom of the slope. | Vehicles will slide or roll to the toe. The slope itself is included in the clear zone width, but a runout area at the toe is required. |
| Steeper than 1:3 | Critical | An errant vehicle will likely overturn. | Critical slopes cannot be counted toward the clear zone. They must be shielded by a barrier if they lie within the clear zone distance. |
Horizontal Curve Correction ($K_{cz}$)
When a horizontal curve is present, the probability of a vehicle running off the road on the outside of the curve increases. The clear zone on the outside of a horizontal curve must be increased by applying a correction factor, $K_{cz}$.
$$\text{Clear Zone on Curve } (CZ_c) = CZ_t \times K_{cz}$$Where:
- $CZ_t$ = Tangent clear zone width (ft), obtained from the standard clear zone table (RDG Table 3.1).
- $K_{cz}$ = Horizontal curve correction factor, obtained from RDG Table 3.2.
- $K_{cz}$ depends on:
- Design Speed
- Radius of the horizontal curve ($R$)
Note: Curve corrections are applied only to the outside of horizontal curves. On the inside of the curve, the standard tangent clear zone width is used.
Roadside Hazard Decisions
When a fixed object or hazard (e.g., utility pole, bridge pier, steep critical slope) is identified within the clear zone, designers must follow the AASHTO Roadside Design Guide’s hazard mitigation hierarchy:
- Remove the obstacle (ideal solution).
- Redesign the obstacle so it can be safely traversed (e.g., flattening a steep slope).
- Relocate the obstacle to a point where it is less likely to be struck (e.g., moving a utility pole outside the clear zone).
- Reduce impact severity by using an appropriate breakaway device (e.g., breakaway sign posts).
- Shield the obstacle with a traffic barrier or crash cushion (if the barrier is less hazardous than the obstacle).
- Delineate the obstacle if no other alternative is practical (least preferred, for low-speed/low-volume areas).
Worked Example: Clear Zone with Curve Correction
Problem Statement
A two-lane rural collector has a design speed of $60\text{ mph}$ and a design ADT of $4,500\text{ vehicles/day}$. The roadway cross-section has a $1:6$ foreslope. A section of this collector features a horizontal curve with a radius of $1,200\text{ ft}$.
- Find the minimum required clear zone width on a tangent section of this roadway.
- Calculate the minimum required clear zone width on the outside of the horizontal curve.
- A large, unshielded concrete bridge abutment is located $28\text{ ft}$ from the edge of the traveled way on the outside of the curve. Determine whether this hazard must be mitigated (relocated, shielded, etc.).
Reference Tables (AASHTO RDG)
- RDG Table 3.1 (Excerpt for 60 mph, ADT 1500–6000, 1:6 Foreslope):
- Tangent Clear Zone ($CZ_t$) = $22\text{ ft}$ to $24\text{ ft}$. Use $24\text{ ft}$ for conservative design.
- RDG Table 3.2 (Excerpt for Curve Correction Factor $K_{cz}$ at 60 mph):
- For $R = 1,200\text{ ft}$ and Design Speed = $60\text{ mph}$: $K_{cz} = 1.3$
Solution
Find the Tangent Clear Zone ($CZ_t$): From the RDG table lookup, using the design speed ($60\text{ mph}$), ADT ($4,500$), and slope ($1:6$ foreslope), the baseline clear zone is:
$$CZ_t = 24\text{ ft}$$Calculate the Curved Clear Zone ($CZ_c$): Multiply the tangent clear zone by the horizontal curve correction factor $K_{cz}$ from the table:
$$CZ_c = CZ_t \times K_{cz}$$$$CZ_c = 24\text{ ft} \times 1.3 = 31.2\text{ ft}$$Evaluate the Hazard:
- The bridge abutment is located $28\text{ ft}$ from the edge of the traveled way.
- The required clear zone on the outside of this curve is $31.2\text{ ft}$.
- Since the bridge abutment ($28\text{ ft}$) is located within the required clear zone ($31.2\text{ ft}$), it represents a hazard that must be mitigated. Because a bridge abutment cannot be removed or relocated easily, it must be shielded using a crashworthy barrier (such as a W-beam guardrail or concrete barrier).
Answer
- Tangent clear zone: 24 ft
- Outside curve clear zone: 31.2 ft
- The hazard lies within the clear zone ($28\text{ ft} < 31.2\text{ ft}$), so shielding or mitigation is required.
Crucial Exam Tips
- Continuous vs. Discontinuous Slopes: If a slope changes from recoverable ($1:6$) to non-recoverable ($1:3.5$) within the clear zone, the non-recoverable portion is traversable but not recovering. You must add the width of the non-recoverable slope to the clear zone calculation to find the total distance to a safe recovery area.
- Curve Correction Applicability: Ensure you only apply the curve correction factor $K_{cz}$ if the hazard is on the outside of the curve. If the hazard is on the inside of the curve, the correction factor is $1.0$ (i.e., use the standard tangent clear zone width).
- Edge of Traveled Way: Always measure the clear zone from the edge of the travel lane (traveled way), not from the edge of the shoulder. The shoulder width is included as part of the clear zone.