Driveway and Access Management
Access management is the systematic control of the location, spacing, design, and operations of driveways, median openings, interchanges, and street connections to a roadway. The primary objective is to balance the competing needs of roadway mobility (moving through traffic safely and efficiently) and land access (providing entry to roadside properties).
Core Principles of Access Management
AASHTO and the Federal Highway Administration (FHWA) identify several foundational principles of access management:
- Limit Conflict Points: Conflicts occur where vehicle paths cross, merge, or diverge. Access management aims to reduce the number of conflict points. For example, replacing a full-access driveway with a right-in/right-out (RIRO) driveway reduces the conflict points from 11 to 2.
- Separate Conflict Points: Spacing driveways and intersections far enough apart allows drivers to address one potential conflict at a time, reducing cognitive load and crash rates.
- Reduce Speed Differentials: Turning vehicles slow down significantly before entering driveways. Large speed differences between through vehicles and turning vehicles increase the risk of rear-end crashes. Access design uses deceleration lanes or appropriate driveway turning radii to minimize this differential.
- Establish a Roadway Hierarchy: Roadways should be classified and designed according to their intended function. Arterials prioritize mobility and should have highly restricted access. Local streets prioritize access and can accommodate frequent driveways.
- Use Non-Traversable Medians: Raised or grassed medians are the most effective tool for managing access on high-volume arterials because they physically prevent left turns across opposing traffic, forcing drivers to make turns at designated, signalized U-turn or median openings.
Driveway Spacing and Corner Clearance
Corner Clearance
Corner clearance is the distance between an intersection and the nearest driveway.
- Significance: Inadequate corner clearance causes severe operational issues. For example, if a driveway is too close to an intersection, vehicles queuing at a red light will block access to the driveway, or a vehicle attempting to turn into the driveway will back up traffic into the active intersection area.
- Design Goal: The corner clearance should ideally exceed the maximum queue length expected at the intersection during peak hours.
Spacing Criteria
Driveway spacing is typically based on the design speed of the roadway. Higher speeds require larger spacing to match the distance traveled during perception-reaction and braking. Standard guidelines recommend:
| Design Speed (mph) | Minimum Recommended Spacing (ft) |
|---|---|
| 30 | 200 |
| 35 | 250 |
| 40 | 300 |
| 45 | 350 |
| 50 | 425 |
| 55 | 500 |
Driveway Vertical Alignment (Profile)
The vertical profile of a driveway must be designed to prevent vehicles from scraping their bumpers or bottoming out. This is governed by the algebraic grade change ($\Delta g$) at the grade breaks.
- Grade Break Limits:
- Low-Volume (Residential) Driveways: Maximum grade change $\Delta g \le 10\%$.
- High-Volume (Commercial/Industrial) Driveways: Maximum grade change $\Delta g \le 8\%$ (ideally $\le 5-6\%$).
- Transitions: For steep driveways, a transition segment (usually $10$ to $20\text{ ft}$ long) must be placed between the roadway cross slope and the main driveway grade to soften the grade break.
Worked Example
A developer is designing a new commercial driveway connecting to a minor arterial street.
- The arterial has a cross slope of $-2.0\%$ (sloping downward toward the curb).
- The sidewalk is $8\text{ ft}$ wide and has a cross slope of $-1.5\%$ (sloping toward the curb).
- The driveway must climb to meet the parking lot elevation.
- The local jurisdiction restricts the maximum grade change ($\Delta g$) at any unsorted grade break to $6.0\%$ for commercial driveways.
- Calculate the maximum permissible positive grade ($g_d$) for the driveway segment immediately following the sidewalk.
- If the design speed of the arterial is $40\text{ mph}$, and the nearest signalized intersection is downstream, determine the minimum corner clearance required based on speed spacing guidelines.
Solution
1. Calculate Maximum Driveway Grade ($g_d$): The grade break occurs at the boundary where the sidewalk pavement meets the driveway pavement.
- Sidewalk grade ($g_{\text{sw}}$) = $-1.5\%$ (downward)
- Driveway grade = $g_d$ (upward, positive)
- Maximum allowed grade change ($\Delta g$) = $6.0\%$
The algebraic grade difference is:
$$\Delta g = |g_d - g_{\text{sw}}|$$$$6.0\% = |g_d - (-1.5\%)|$$$$6.0\% = g_d + 1.5\%$$$$g_d = 6.0\% - 1.5\% = +4.5\%$$The driveway segment immediately following the sidewalk cannot exceed a slope of $+4.5\%$ without violating the grade break limit.
2. Determine Minimum Corner Clearance:
- Design speed = $40\text{ mph}$.
- According to the driveway spacing table, the minimum spacing required to prevent conflict overlap and manage speed differentials at $40\text{ mph}$ is $300\text{ ft}$.
- Therefore, the driveway must be located at least $300\text{ ft}$ away from the intersection curb line.
References
- A Policy on Geometric Design of Highways and Streets (AASHTO Green Book), 7th Edition, 2018, Section 9.10.
- TRB Access Management Manual, 2nd Edition, 2014.