Two-Way Bridge Clearance Calculator

Enter six decimal-foot readings for each travel direction. Choose one clearance for the whole bridge or calculate a separate clearance for each direction.

Why this calculator: the MMUTCD changed

Michigan’s advance-warning threshold increased by 1 foot: from below 13 ft 6 in to below 14 ft 6 in. A bridge with 14 ft of actual clearance falls within the new advance-warning threshold even though it did not fall within the old threshold.

Low-clearance signing: earlier MMUTCD versus 2025 MMUTCD
Provision	Earlier MMUTCD · Section 2C.27	2025 MMUTCD · Section 2C.25
Advance warning	W12-2 required below 13 ft 6 in.	W12-2 required below 14 ft 6 in, or below the statutory maximum vehicle height plus 12 inches, whichever is greater. Michigan’s 13 ft 6 in statutory height makes the threshold 14 ft 6 in.
Displayed clearance	Guidance uses 2-inch increments without exceeding actual clearance.	Guidance uses 1-inch increments without exceeding actual clearance.
Measurement coverage	Guidance includes the travel lanes and a point 2 feet off the edge of the traveled way to account for vehicle overhang.	Michigan guidance applies minimum-clearance determinations over the complete usable shoulder width.
Overhead signs	Rectangular low-clearance signing recommended at structures with clearance of 14 ft or less.	W12-2a or W12-2b overhead signing recommended at 14 ft 6 in or less. W12-2b identifies a specific low-clearance lane or shoulder; the advance legend should match the lowest overhead value.

What this calculator helps with: Check the new advance-warning threshold using actual field measurements, then convert decimal feet to a sign legend using whole-inch rounding. The selected logic determines whether both sides share one bridge minimum or each direction uses its own minimum.

The frost allowance existed in both editions. Both include guidance for a reduction of up to 3 inches where frost action affects clearance. This calculator applies a fixed 3-inch allowance after checking the actual minimum against the advance-warning threshold.

Compare the earlier Section 2C.27 excerpt, 2025 Section 2C.25 excerpt, and MDOT’s Michigan provisions for Section 2C.25 (PDF, page 13). See the compliance dates below.

How this calculator works

Choose the calculation logic

Logic 1 uses the lowest of all 12 readings for both sides of the bridge. Both sides use the same sign legend and threshold decision.

The 14 ft 6 in threshold is checked BEFORE the 3-inch reduction.

Find the actual minimum. Use the lowest of all 12 readings across both travel directions and both bridge ends. Apply that minimum to both sides.
Check whether signing is required. Compare that actual minimum with 14 ft 6 in. Only a measurement below 14 ft 6 in triggers signing in this calculator. Exactly 14 ft 6 in or higher does not trigger signing.
Calculate the sign legend. Subtract the 3-inch frost/resurfacing allowance from the actual minimum, then round down to a whole inch. This adjusted value determines the displayed legend; it does not change the threshold decision.

Logic 1 example: If the directional minimums are 14 ft 2 in and 14 ft 4 in, both sides use 14 ft 2 in as the actual minimum and display 13 ft 11 in after the 3-inch reduction.

Example: An actual minimum of 14 ft 7 in produces a calculated legend of 14 ft 4 in after the reduction. Signing is not required by this calculator’s threshold because the actual minimum is above 14 ft 6 in.

Two-way layout: Each travel direction uses six readings: EM left, crown, and EM right at the entering and exiting ends of the bridge. See the RCOC Low Clearance Signs guidance.

Quick reference

What the MMUTCD says

Section 2C.25

Advance warning: Use a W12-2 in advance when actual vertical clearance is less than 14'-6", or less than 12 inches above the statutory maximum vehicle height, whichever threshold is greater.
Displayed clearance: Show the clearance to the nearest inch without exceeding the actual clearance. A reduction of up to 3 inches should be used where frost action can change the clearance.
Overhead signs: W12-2a or lane-specific W12-2b signs may supplement the advance sign. Use additional overhead signs where clearance varies greatly, and keep the advance legend consistent with the lowest overhead value.

MDOT Design Advisory DA 2026-02 — 2025 MMUTCD (PDF)
Issued April 20, 2026. The advisory includes Table 1B-1 and its five-year target compliance periods for low-clearance advance signs and variable-clearance overhead signs. These items are coordinated statewide and are excluded from the August 7, 2026 project-use requirement.

August 7, 2026

Michigan project-use date: The 2025 MMUTCD applies to MDOT-let projects whose contract documents reference its latest version beginning with this bid letting. Table 1B-1 items follow separate statewide coordination.

January 18, 2029

Low-clearance target date: Five years after the 11th Edition's January 18, 2024 effective date for Section 2C.25, Paragraph 1 (W12-2 advance-sign requirement) and Paragraph 8 (variable-clearance overhead-sign guidance).

Sources: MUTCD Table 1B-1 and the MDOT Design Advisory linked above.

Field measurements

A two-way bridge uses 12 readings total—six for each travel direction.

MDOT Form 1190 (BOBS) — Structure Clearance Measurements (PDF) · February 2026 revision

Direction 1

Travel Direction 1

6 readings

Entering end

EM left (ft) —

Crown (ft) —

EM right (ft) —

Exiting end

EM left (ft) —

Crown (ft) —

EM right (ft) —

Direction 2

Travel Direction 2

6 readings

Entering end

EM left (ft) —

Crown (ft) —

EM right (ft) —

Exiting end

EM left (ft) —

Crown (ft) —

EM right (ft) —

Bridge-wide results

Logic 1: Both sides use the lowest of all 12 readings. Each sign legend includes the 3-inch frost/resurfacing allowance and is rounded down to a whole inch.

Enter all 12 readings to calculate one height for both sides.

Important: “Sign legend” and “sign required” are separate. The standard threshold is checked against the actual measured minimum before the 3-inch allowance. A calculated sign legend below 14 ft 6 in does not itself trigger signing.