This calculator helps traffic engineers quickly evaluate whether a left-turn or right-turn lane is warranted at an intersection approach along two lane highways, using AASHTO lane warrant guidance.
How to calculate left turn or right turn lane warrant:
- Select the lane type: Left Turn or Right Turn.
- Enter the design parameters:
- Design Speed – the speed used for geometric and traffic design. Use Design Speed if Available. If not, use the legal speed limit.
- Peak Hour Volumes – enter opposing, advancing (total volume on approach) volumes as applicable.
- Turning percentages – for left turns, enter the percentage of vehicles turning left. For Right turns, enter the number of vehicles turning right per hour.
- Click Calculate to see if the lane warrant is met.
This tool is designed for preliminary screening and speeds up manual lookups from AASHTO tables.
Note for Left-Hand Drive (LHD) networks: This calculator is based on Right-Hand Drive (RHD) traffic. For LHD networks, simply swap the lane directions when entering data: Use the Right-Turn Lane Warrant values for the left-turn lane, and vice versa. Treat the volumes normally assigned to the right turn as left-turn volumes. Following this approach will allow the tool to provide consistent preliminary screening for LHD sites without confusion.
Turn Lane Warrant Calculator
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Disclaimer: This calculator is intended for preliminary design and screening only. Users must verify all results against local standards and AASHTO guidance. Arterials accepts no liability for design decisions made using this tool. For official submissions, always cross-check with published AASHTO tables and field conditions.
Warrant Reference Tables
Left-Turn Lane Volume Warrants (AASHTO Method)
- Shows the minimum advancing and opposing volumes required for a left-turn lane at different operating speeds and left-turn percentages.
- Use this table to verify results from the Turn Lane Warrant Calculator.
| Advancing Volume/Hour | ||||
|---|---|---|---|---|
| Opposing Volume (veh/h) | 5% Left Turns | 10% Left Turns | 20% Left Turns | 30% Left Turns |
| 60 km/h Operating Speed | ||||
| 800 | 330 | 240 | 180 | 160 |
| 600 | 410 | 305 | 225 | 200 |
| 400 | 510 | 380 | 275 | 245 |
| 200 | 640 | 470 | 350 | 305 |
| 100 | 720 | 515 | 390 | 340 |
| 80 km/h Operating Speed | ||||
| 800 | 280 | 210 | 165 | 135 |
| 600 | 350 | 260 | 195 | 170 |
| 400 | 430 | 320 | 240 | 210 |
| 200 | 550 | 400 | 300 | 270 |
| 100 | 615 | 445 | 335 | 295 |
| 100 km/h Operating Speed | ||||
| 800 | 230 | 170 | 125 | 115 |
| 600 | 290 | 210 | 160 | 140 |
| 400 | 365 | 270 | 200 | 175 |
| 200 | 450 | 330 | 250 | 215 |
| 100 | 505 | 370 | 275 | 240 |
Note: All values are sourced from AASHTO’s A Policy on Geometric Design of Highways and Streets and FHWA guidance. Confirm all design decisions with the latest published tables.
Right-Turn Lane Threshold Volumes (AASHTO Method)
- Lists the approach and right-turn volumes required to justify a right-turn lane.
- Reference this table when evaluating right-turn lane warrants in preliminary design or TIA reports.
| Approach Volume / Right-Turn Volume (DHV) | |
|---|---|
| Approach Volume (veh/h) | Right-Turn Volume (veh/h) |
| 100 | 100 |
| 200 | 94 |
| 300 | 80 |
| 400 | 68 |
| 500 | 54 |
| 600 | 40 |
| 700 | 40 |
Note: All values are sourced from AASHTO’s A Policy on Geometric Design of Highways and Streets and FHWA guidance. Confirm all design decisions with the latest published tables.
Including Results in Traffic Impact Assessment (TIA) Reports
To document the lane warrant evaluation in your TIA:
- Include a subsection in the Traffic Analysis chapter titled “Turn Lane Warrants.”
- List input parameters used in the calculator (operating speed, volumes, % turning).
- Present the results in a concise table or figure showing:
- Lane type
- Threshold volume
- Actual volume
- Warrant met / not met
- Reference your source tables: FHWA / AASHTO guidelines (you can link the tables in your report).
- Optionally include a screenshot or exported values from the calculator for clarity.
- Provide a note that preliminary results are included for screening, with final design subject to local review.
For convenience, you can use our free TIA Report Template to include lane warrant tables and commentary consistently.
FAQs – Turn Lane Warrant Calculator
Can I use this calculator for multilane highways or urban arterials?
No. The current thresholds are based on AASHTO guidelines for two-lane highways. For multilane roads or urban arterials, lane warrant volumes and design criteria may differ. Always consult local standards and AASHTO guidance.
Are the operating speed and traffic volumes required in the tool field measurements or estimates?
Use the 85th percentile speed for operating speed and design hourly volumes (DHV) for traffic volumes. These are standard inputs for preliminary screening.
What do I do if my site has unusual geometry, signalized intersections, or high pedestrian activity?
This calculator is preliminary only. Complex conditions may require a full engineering analysis using AASHTO, MUTCD, or local guidelines.
Can I rely on the results for official TIA reports?
Use these results as a screening tool. Always cross-check with published tables, local standards, and professional judgment.