One of the most common questions in traffic impact assessments (TIAs) is whether to propose turning lanes, acceleration lanes, or deceleration lanes. While adding lanes may seem like a straightforward solution to congestion, proposing them without justification can create unnecessary cost, invite reviewer comments, or even compromise safety.
In this post, we’ll explain when to propose auxiliary (Right Turn) lanes, differentiate between intersection vs site driveway warrants, and show how collision history, HCM/FHWA guidelines, and capacity analyses influence lane design. We’ll also link you to the Arterials Turning Lane Warrant Calculator and Sightline / Sight Triangle Tool to make your workflow faster and more defensible.
Why We Can’t Just Add Lanes
Adding turning or auxiliary lanes without justification can backfire:
- Cost and construction impact – Additional lanes increase pavement, drainage, and right-of-way requirements.
- Reviewer scrutiny – TIAs are closely reviewed. Proposing lanes “just to be safe” or without a warrant often triggers reviewer questions.
- Safety concerns – Misaligned or unnecessary lanes can introduce conflict points, reduce driver predictability, and impact pedestrians.
Key takeaway: Every lane must have a quantifiable rationale, based on traffic volumes, operational warrants, or collision history.
Understanding Lane Warrants
Left-Turn Lane Warrants
The Highway Capacity Manual (HCM 6th Edition) and FHWA guidelines provide thresholds for exclusive left-turn lanes for signalized and unsignalized (stop controlled):
- Volume-based warrants: Left-turn lanes are warranted when left-turn volumes exceed thresholds that cause queuing in through lanes or reduce intersection operations below acceptable LOS. The warrant does however change with the design speed of the roadway.
- Collision-based warrants: High left-turn crash rates can justify lanes even if volume warrants aren’t met.
Our Turning Lane Calculator Tool lets you quickly check left-turn lane warrants based on approach volumes and desired LOS.
Right-Turn Lane Warrants – Intersections vs Site Driveways
Right-turn lane criteria differ depending on whether you’re at a signalized/unsignalized intersection or a site access driveway:
Right turn lanes at Intersections
- Right-turn lane warrants are often volume-based, similar to left-turn lanes, and included in HCM/FHWA guidance.
- If volumes are low but through-lane capacity is critical, intersection capacity analysis using software like Synchro may justify a lane even if the volume warrant isn’t met.
- This is especially true on major arterials where even a modest queue in a right-turn lane can degrade overall intersection operations.
Right Turn lanes at Site Driveways
- Right-turn lanes at private driveways usually rely on volume warrants and capacity analysis.
- Factors include: expected turning volumes during peak hours, site trip generation, and effects on through-traffic operations.
- In some cases, Synchro (simtraffic) or VISSIM simulations are used to evaluate whether a right-turn lane is necessary to maintain acceptable LOS on the mainline.
Acceleration and Deceleration Lanes
Acceleration and deceleration lanes at driveways or intersections are generally proposed based on:
- Design speed of the main roadway
- Vehicle volumes entering/exiting
- Expected delay to through traffic
The AASHTO Green Book and FHWA provide formulas for calculating required lane lengths to safely accommodate vehicles without disrupting flow.
Collision History Considerations
Even if volume-based warrants aren’t met, crash history can justify auxiliary lanes:
- High left-turn crash rates → left-turn lane warranted.
- Rear-end collisions at driveways → deceleration (right turn) lane may be justified.
This approach strengthens your TIA defensibility while improving safety.
Integrating Sightlines and Visibility
Lane proposals also intersect with driver sight distance requirements:
- Inadequate sight triangles can negate the benefits of new lanes.
- Using a Sightline / Sight Triangle Tool ensures proposed lanes are safe for motorists and pedestrians.
Check out our Sightline Tool to quickly evaluate minimum visibility requirements at intersections or driveways.
Best Practices for TIA Lane Proposals
- Check warrants first – HCM, FHWA, and intersection/driveway-specific guidelines.
- Document your assumptions – traffic volumes, peak hours, design speed, and analysis tool used.
- Consider collision history – justify lanes even if volume warrants aren’t met.
- Use intersection capacity analysis when needed – especially for site driveways or high-volume approaches.
- Verify sight distances – lanes without visibility can reduce safety.
- Leverage tools – Arterials’ Turning Lane Calculator and Sightline Tool streamline defensible designs.
Bottom Line
Proposing turning or auxiliary lanes isn’t just about moving vehicles faster. It’s about making evidence-based, safe, and reviewer-defensible design decisions. By distinguishing intersection warrants vs driveway analysis, considering collision history, and verifying sight distances, you can ensure your TIA recommendations are sound and cost-effective.
