Measure bridge width between inside faces of curbs or rails for clear roadway, and record deck width out-to-out across the deck.
Need a clear, site-tested way to record bridge width that lines up with inspection forms and inventory rules? This guide shows the field method, the terms that matter, and the common traps that cause inconsistent numbers. You’ll see where to place the tape, what to include or exclude, and how to document the result so anyone can repeat it. If you’re asking how to measure bridge width under time pressure, the steps below keep you fast and consistent.
How to Measure Bridge Width: Step-By-Step
Bridge projects use two measurements most often: clear roadway width and deck width. Clear roadway width is the tightest distance between the inside faces of traffic barriers, rails, or curbs on the traveled portion. Deck width is the full out-to-out width of the deck. Both numbers matter: one speaks to operations and permits, the other feeds inventory, ratings, and performance reporting.
Tools You’ll Want
- 100-ft fiberglass tape or a laser distance meter with verification tape
- Chalk or tape marks for face-of-rail alignment
- Level or straightedge to project faces inward when rails have flares
- Camera or phone for photos at each measuring point
- Notebook or digital form with fields for deck width and clear roadway width
Step 1: Set Reference Faces
Stand on the deck near mid-span where there’s no flare or thickened post. Identify the inside face of each barrier, rail, or curb. If the face is battered or has a slope, use a level to project a vertical plane and mark the line on the deck. That line is your reference.
Step 2: Take The Clear Roadway Width
Hook the tape on the near reference line and stretch to the opposite line at a right angle to traffic. Pull snug, hold level, and read the distance to the nearest tenth. Repeat at quarter points of the span to check consistency. Record the smallest value as the clear roadway width.
Step 3: Take The Deck Width (Out-To-Out)
Move to the outermost edges of the deck slab or wearing surface. Measure from edge to edge at a right angle to traffic. If parapets overhang the slab, keep measuring to the outer deck edge, not to the outside of the parapet. Note any cantilevered sidewalk or utility bay.
Step 4: Capture Sidewalks And Shoulders
Measure each sidewalk clear width between the inner face of the parapet and the curb or rail that bounds the walkway. Measure shoulder widths between the traveled lane and the inside face of the rail or curb. List each side separately.
Step 5: Document Conditions
Add notes on flares near approach transitions, removable railing, work zones with temporary barriers, or staged construction that narrows the path. Snap photos from mid-span and at each quarter point showing the tape placement and reference faces.
Bridge Width Terms At A Glance
This table summarizes the terms you’ll see in design guides, inspection manuals, and inventory coding. It keeps your tape on the right faces and your notes consistent across crews.
| Term | Where You Measure | Include / Exclude |
|---|---|---|
| Clear Roadway Width | Inside face to inside face of curbs, rails, or barriers | Include lanes and shoulders inside faces; exclude sidewalk bays |
| Deck Width (Out-To-Out) | Outer deck edge to outer deck edge | Include sidewalks and overhangs; exclude gutter drip beyond deck |
| Curb-To-Curb Width | Inside curb to inside curb | Include traveled way and gutter to curb face; exclude sidewalks |
| Between Barriers | Inside barrier face to inside barrier face | Same as clear roadway when no curb is present |
| Sidewalk Clear Width | Walkway inner face to guard or curb | Exclude handrail post protrusions into the travel line |
| Shoulder Width | Lane edge line to inside rail or curb | Measure each side separately; round to the tenth |
| Approach Roadway Width | Approach cross-section, off the structure | Not a deck measure; used for consistency checks |
| Posted Width | Operational width usable by traffic | Reflects temporary barriers or staging when present |
| Usable Width In Staging | Between temporary devices | Exclude closed lanes and work buffers |
Measuring Bridge Width On Site: Quick Setup
Pick a calm weather window to cut wind sway on the tape. Place cones or a spotter if traffic is live. For long spans, a laser meter can speed cross-section checks, but confirm with a tape at least once. When the rail has recesses or pickups, aim for a smooth face location so the reading repeats later.
Lane Lines And Skews
On skewed bridges, measure perpendicular to traffic flow, not square to piers or fascia beams. If lane lines wander near joints, keep the tape perpendicular to the centerline station you choose. Note the station so future crews duplicate your spot.
Where Errors Sneak In
- Reading to parapet caps: Caps extend beyond the deck. The out-to-out reading belongs at the deck edge.
- Measuring clear width at a flare: Flares widen or pinch. Use the smallest repeatable width along the main span, away from flares.
- Mixing curb-to-curb with between-barriers: These can differ on bridges with mountable curbs.
- Forgetting sidewalk bays: Deck width includes them even when closed to the public.
- Using approach width on the deck: Approach width is a check, not the deck value.
Why Two Numbers Matter
Clear roadway width ties to operations, oversize permits, and warning signs. Deck width feeds asset records, ratings, and national reporting. Many agencies center their inventory forms on these two fields, so write them cleanly and keep photos attached to the record.
Anchoring Your Numbers To Standards
Inventory rules use consistent definitions. If you want a single source to confirm your fields, the FHWA Recording and Coding Guide (Items 51 and 52) lays out “Bridge Roadway Width, curb-to-curb” and “Deck Width, out-to-out” with coding to the tenth. National performance reporting also picks up deck width from that same item under 23 CFR 490. Link your photos and sketches to those fields so auditors can trace every number back to the face you measured.
Field Method That Sticks During Audits
Write your location, station, and temperature on the sheet. Temperature matters when tapes expand across long pulls. Take three cross-section readings: at mid-span and at the quarter points. Use the smallest clear roadway value and the modal deck value unless the deck edges taper, in which case use the smaller reading. Note any transient setup, such as movable barrier shifts.
Photos That Prove Your Tape Placement
Take one photo square to each barrier face showing the tape hook against the mark. Take one across the width showing the tape path. If the rail face is irregular, add a close-up with a level establishing the face line. Label the images with the station and the value you read.
how to measure bridge width In Special Cases
Some bridges add steps. The notes here keep the same logic: pick the correct faces, then record the number that matches the term you’re after.
Flared Railings Near Transitions
Measure clear roadway where the rail is parallel to traffic. If flares reach into the span, run several readings and select the smallest that traffic must pass through.
Temporary Barriers And Staged Work
When a work zone narrows the deck, record the operational width between temporary devices in your log. Keep the permanent clear roadway width and deck width in the asset record as separate lines so the history remains clean.
Sidewalks And Shared-Use Paths
For pedestrian or shared-use bridges, measure the inside clear width of the path between the walk barriers. On mixed-traffic bridges with sidewalks, keep sidewalk clear width as a separate entry from the roadway numbers.
Table Of Measurement Scenarios
Use this shortcut when conditions differ from the plain case. It shows where to put the tape and what to record so the result tracks back to a clear term.
| Scenario | Tape Placement | Record As |
|---|---|---|
| No Curbs, Two Barriers | Inside barrier face to inside barrier face | Clear roadway width |
| Raised Curbs With Sidewalks | Inside curb face to inside curb face | Curb-to-curb and deck width |
| Parapets With Overhangs | Outer deck edge to outer deck edge | Deck width (out-to-out) |
| Temporary Barrier Stage | Between temporary device faces | Operational width (log entry) |
| Flared Rails At Approaches | Parallel segment, smallest repeatable span width | Clear roadway width |
| Shared-Use Path Bridge | Inside walk barrier to inside walk barrier | Path clear width |
| Skewed Deck | Perpendicular to traffic centerline | Clear roadway and deck width |
| Split Decks (Twin Structures) | Each deck measured separately | Two records: each deck’s widths |
Reporting And Notation That Others Can Follow
Write numbers to the tenth in feet or to centimeters, depending on your form. Keep units consistent through the sheet. If your district prefers metric on deck width and feet on clear roadway, write both once and label each number. Add a small sketch with arrows showing the faces measured. Attach photos to the line item for each width.
Checks That Catch Mistakes Before They Spread
- Compare to last cycle: If the deck wasn’t widened, the out-to-out value should match prior records within normal tape tolerance.
- Compare to approach width: A huge jump can signal a curb-to-curb vs barrier-to-barrier mix-up.
- Watch for lane drops: If striping changed, the deck width won’t change, but the clear roadway can if barriers moved.
- Re-measure outliers: Anything that stands out gets a second pass before you leave the site.
Where This Data Gets Used
Permits teams look at clear roadway width to judge over-width loads. Asset teams and reporting programs pull deck width for national summaries. That’s why a crew that knows how to measure bridge width the same way every time saves back-and-forth later. Clean notes today prevent guesswork during audits, project scoping, and ratings.
Quick Reference: The Two Go-To Numbers
Clear roadway width: smallest inside-face distance between rails, barriers, or curbs on the traveled way. Deck width: outer deck edge to outer deck edge, including sidewalk overhangs. Capture both at mid-span and quarter points, take the smallest clear width, and document your station and photos.