Can Drone Inspections of County Bridge Undersides Avoid Lane Closures?

How do county bridge underside drone inspections affect lane closures?

County bridge underside drone inspections reduce closure time for many checks, but they do not remove the need for closures entirely. Your question has two parts. The first is whether a drone can view the underside of a span without a person on the deck or a shut lane for the aircraft itself. The second is whether closures can be avoided altogether, and that depends on what each inspection finds and what the rules require.

Federal guidance supports drone use for this work. A 2021 FHWA report on routine bridge inspections using unmanned aerial systems describes drone imagery of structural areas underneath the deck that an under-bridge inspection truck could not reach. A lane closure is usually there to put a person close to the steel and concrete, so a drone changes how much of that close look you need, not whether you need it at all.

The schedule question matters as much as the equipment question. Spring work on creeks that flood every year is shaped by the interval rules and by flood events, so the first task is to sort your 40 spans by what each one legally requires. A spreadsheet with span ID, creek name, last inspection date, interval, flood history and whether a truck visit was needed last time will show you the real workload, and then you can decide which visits a drone can take over.

How many bridges do counties own, and is a 40-span program unusual?

According to a 2019 National Association of Counties analysis of 2018 National Bridge Inventory data, the country has about 602,621 bridges, and counties own 38% of them. That analysis is dated, so use it as a baseline and check the current National Bridge Inventory for newer counts before you present figures to a commission.

State figures show the same pattern. The Kentucky Association of Counties, citing the National Bridge Inventory, reports that counties own and maintain 5,051 bridges, or 35% of nearly 15,000 in the state. The Iowa Department of Transportation 2024 Annual Bridge Report counts 18,225 county-owned bridges out of 23,684 total.

A 40-span program is therefore ordinary for a county, not an outlier. What matters more is how many of those spans cross water with a flood history, because those spans drive the spring schedule and the scour rules.

How often does a bridge underside need to be inspected?

Under the 2022 update to the federal bridge inspection rule, the default routine interval is 24 months and the default underwater interval is 60 months, according to a North Carolina DOT memo summarizing the change. The memo cites section 650.311 of 23 CFR Part 650, Subpart C. Its title refers to a risk-based method, so your state's approved plan controls the final interval for each span.

Underwater intervals shorten when the condition is poor. Michigan DOT guidance says an underwater interval drops to 24 months or less when underwater condition is serious or worse, meaning NBI item 60 or 62 is rated below 4. That guidance dates from 2013, so confirm it against the current rule before you rely on it.

Scour is the flood connection. Scour-critical structures follow a scour action plan, and a scour inspection is due after a flood event, according to the same Michigan guidance. For a creek that floods every spring, this rule can matter more than the routine calendar, because the spring visit may be a required follow-up rather than a scheduled one. Verify these thresholds against the current eCFR text for 23 CFR 650.311 before you rely on them.

• Routine default: 24 months
• Underwater default: 60 months
• Underwater serious or worse (NBI item 60 or 62 below 4): 24 months or less
• Scour-critical structures: scour action plan, plus an inspection after a flood event

Can a drone see the underside of a bridge without a truck?

In many cases, yes. A 2019 FHWA technical brief says current practice favors multirotor aircraft over fixed-wing aircraft for bridge inspection. The 2021 FHWA report describes imagery of structural areas underneath the deck, which is the core gain over a truck that cannot reach those areas.

Drone imagery is a record of conditions, not a hands-on test. Inspectors still judge cracking, section loss, bearing condition and debris, and some sources say certain items require physical contact. A drone can show where to look and what has changed since the last visit, which can cut the number of truck setups, but it does not replace a close look at a flagged item.

Treat the drone as a way to triage the underside and document it, then send the truck where the imagery raises a question. Ask any drone provider for sample imagery from a span like yours before you commit, and check whether the images show the bearings and pier caps clearly enough for your inspectors to work from.

How much lane closure time does a drone inspection save?

Published time figures are useful but mostly modeled or vendor-reported, so use them as direction rather than promises. An Ohio academic case study modeled an 8-hour traditional lane closure against a 4-hour drone-assisted closure for each inspection. The study treated those values as assumptions, not measured results.

A 2016 trade article reported an AASHTO-related survey in which drone inspections took about two hours against about eight hours for manual inspection. That figure is old and does not describe the spans, traffic or closure rules behind it. Vendor case studies in this area claim larger cuts, but no independent audit of those claims was found.

Expect the saving to come from fewer closure hours per span and fewer truck setups, not from zero closures. Every finding that needs a hands-on check brings the truck, the traffic control and the lane closure back. Record whether each closure was for the truck, the traffic control or the flight itself, because those three carry different savings. The real gain depends on how often a finding needs that follow-up on your bridges.

What can a drone not inspect on a bridge?

The limits are where county programs often get surprised. Federal law requires an inspection team leader on site during the inspection, according to an FHWA summary of the National Bridge Inspection Standards. A drone cannot run an inspection alone, so a qualified person must be present and in charge.

An academic model of UAS bridge inspection notes that some findings need a follow-up with an under-bridge truck and a lane closure. Build that follow-up into the workflow from the start, rather than treating it as an exception. Drone data also does not replace the inspector's professional judgment on condition ratings.

Weather and water limit flights as well. Wind, rain and high water affect when an aircraft can fly and where it can launch. The sources reviewed did not give flight-day statistics for this, so build a weather and access log from your own spring records.

Do drone bridge inspections need an FAA waiver?

According to a trade summary of FAA rules, small drones under 55 pounds fall under Part 107. Under current rules, flights beyond visual line of sight need an individual FAA waiver, and a bridge crew should plan for that before scheduling a long creek corridor.

The rule that would change this is the beyond visual line of sight rule, Part 108. The trade summary says it went to White House review on July 10, 2026, and the FAA hoped to publish before year end. It had not been published as of mid-September 2026, so check the FAA site for current status before you write a contract scope around it.

The sources did not confirm current rules on flying small drones over people. Do not state a rule on that point without checking the FAA source, and plan traffic control around any live lane regardless of the aircraft rules.

How do spring creek floods change when and how spans get checked?

Flood season matters for access more than for the rule itself. Launch sites, recovery points and inspector access on a creek bank can be under water for part of the year, so pick launch and landing areas above the expected waterline and confirm them on a dry-day walk. Creek banks also soften after heavy rain, so a crew walking the site after a flood needs its own footing check.

The scour rule follows the river rather than the calendar. Once a flood event passes, scour-listed structures may need an inspection, which a drone can support with a quick underside look after the water drops, if the aircraft can fly safely that day. The search did not surface published figures on flood-season inspection timing, so the schedule has to come from your own flood records.

Keep a flood-event log for each span, with the date and any gauge reading you have, and tie each drone flight to a specific trigger: a flood, a scheduled interval, or a flagged finding. That keeps the program defensible in a review and keeps flights that do nothing off the calendar.

Who staffs and services a drone inspection program, and how does leasing work?

Published staffing and injury figures for county bridge inspection were not available in the sources reviewed, so estimate crew hours from your own closure records. Keep the team leader and the drone pilot as separate roles, since the team leader on site is a federal requirement and the pilot needs its own qualifications.

We are a vendor-neutral integrator. We select the aircraft that fits your spans and conditions across manufacturers, lease it through financing, deploy and integrate it, train your staff, and service every unit under our service plan with regional service engineers. The first step is a free site assessment, and the lease quote follows that assessment.

For a county, a practical first step is a pilot on a handful of spans over flood-prone creeks, with closure times and truck returns logged from day one. That gives you measured results on your own bridges, which is the evidence a commission will ask for.