11Roads, bridges, tunnels and the hole in your street

The short version: A road is a layered structure, and potholes start when water gets under the top layer and freezing and traffic finish the job. In the US, bridges have been inspected under federal standards since 1971, today usually every two years, and graded good, fair or poor by their weakest part. Before anyone digs in a US street, call-before-you-dig rules have utilities mark what is buried there, and many cities restrict digging in freshly paved streets. Boston carries the scars and lessons of the Big Dig, whose tunnel ceiling collapse in 2006 killed a driver, and is trying to reach zero traffic deaths by 2030.

What is under the asphalt?

A street looks like one slab, but it is a stack. In the language of pavement engineers, an asphalt road is a flexible pavement: a bound surface layer sitting on a base course of angular crushed rock, which sits on the subgrade, the native soil the road is built on.⁷⁵ The surface keeps water out and gives tires grip, and the base spreads each wheel load over a wider area so the soil underneath is not overloaded. Angular rock is used because sharp-edged stones interlock where round ones would roll.

That division of labor explains most road failures. As long as the surface stays sealed, the base and soil stay dry and strong. Once water gets through, the whole stack starts to fail from below.

Why are potholes worse in spring?

Potholes form in a sequence.⁷⁵

1. Cracks. The surface ages and cracks under traffic and weather.
2. Water gets in. Rain and snowmelt seep through the cracks into the base and subgrade, and the soil softens.
3. Pumping. Each passing tire pushes down on the wet layer and squeezes water, carrying fine soil particles with it, out through the cracks. The support under the surface thins out.
4. Feedback. Less support means more flexing, more flexing means more cracks, and more cracks let in more water, until the surface breaks away.

Cold climates add a sharper tool. "Water expands when it freezes, and it does so with tremendous force." Thin layers of water between pavement and base freeze and grow into formations called ice lenses, which lift the pavement. "When those lenses thaw out, all the ice that was supporting the pavement goes away, creating voids."⁷⁵ The surface is then bridging over empty space, and traffic breaks it. That is why potholes are worst during the spring thaw.⁷⁵ A freeze-thaw cycle is one round of that freezing and melting. So a winter that swings above and below freezing many times can do more damage than one that stays steadily cold.

Minnesota DOT research at its Mn/ROAD test road found that during a partial thaw, pavement layers can sit "in a saturated, weakened state" from trapped water, and that the stiffness of the aggregate base drops sharply just after the thaw passes through it.²⁴⁸ In Boston, at a May 8, 2026 City Council hearing, Chief of Streets Nick Gove said the average pothole case was resolved in 18 hours and over two-thirds within 24 hours; figures shown at the hearing counted over 4,000 pothole-repair requests in East Boston over five years.²⁴⁹

This also explains why a cold patch put down in March can fail by May. A cold patch is a temporary repair: it plugs the hole in the surface but cannot fix the wet, weakened layers underneath.

How it changed: why US bridges get inspected

US bridge inspection is a federal system built up over decades:

The trigger was the Silver Bridge, which carried US 35 over the Ohio River between Point Pleasant, West Virginia, and Gallipolis, Ohio. It collapsed on December 15, 1967, killing 46 people; the cause was the sudden fracture of a single eyebar in a suspension chain, starting from a defect about 0.1 inch (2.5 mm) deep.²⁵³ Congress responded with a bridge inspection section in the Federal-Aid Highway Act of 1968, and the National Bridge Inspection Standards followed on April 27, 1971.²⁵³

How often are bridges inspected?

Under the current US rule, the baseline is a routine inspection every 24 months. A bridge in good shape that meets set criteria can go up to 48 months. A bridge with a relevant component rated 3 ("serious") or lower on a 0-to-9 scale gets inspected every 12 months.²⁵¹

Inspectors rate each major part of a bridge separately: the deck (the surface you drive on), the superstructure (the beams or girders that hold up the deck), the substructure (piers, abutments and foundations) or, for a buried pipe-like crossing, the culvert. The bridge's overall condition is set by the lowest of those ratings:²⁵²

These definitions come from the January 2017 Pavement and Bridge Condition Performance Measures rule.²⁵² A "poor" rating does not mean "about to fall down": it means at least one major part has deteriorated badly enough to need serious attention, and the 12-month interval for low ratings is how the system watches those bridges more closely. Older news stories use the term "structurally deficient." How exactly that older label maps onto "poor," and when federal reporting switched, is not confirmed by our sources.

The US also has a separate federal program for road tunnels, the National Tunnel Inspection Standards, published in the Federal Register on July 14, 2015.²⁵⁴²⁵⁵

In Boston: the Big Dig ceiling collapse

The Big Dig is the common name for Boston's downtown highway and tunnel megaproject, which includes the I-90 connector tunnel.²⁵⁶

It replaced the deteriorating six-lane elevated Central Artery (I-93) with an underground expressway, built a four-lane tunnel under Boston Harbor and added the Leonard P. Zakim Bunker Hill Memorial Bridge. Construction ran from 1991 to 2007, and FHWA puts the cost at $14.8 billion, $7 billion of it federal.²⁵⁷ Other accounts give $14.6 billion at completion in December 2007, and about $21.9 billion once interest is counted.²⁵⁸

On July 10, 2006, about 12 tons of concrete ceiling panels fell in the I-90 connector tunnel, killing Milena Del Valle, 39. Each panel weighed 4,600 pounds (about 2,090 kg), and the anchors holding the ceiling up were set with epoxy adhesive.²⁵⁶

The National Transportation Safety Board named the likely cause as the "use of an epoxy anchor adhesive with poor creep resistance."²⁵⁶ Creep is slow deformation of a material under a constant load. The epoxy did not snap; under constant load it gave way a little at a time, and the anchors slowly pulled out until the panels fell. The fast-set epoxy was supplied by Powers Fasteners and installed by Modern Continental.²⁵⁶

The lesson is about load that never lets up. A ceiling panel puts the same weight on its anchors every second of every day, so a material that holds in a short test can still fail over a long time. The MBTA's 2025 fallen ceiling tiles at Harvard and Park Street stations, covered in How the subway and the bus actually run, are a reminder that overhead hazards are not only a highway problem.²²⁴

Tunnels also need light and air. In Boston, the MBTA's power network feeds tunnel lighting and ventilation fans.⁵¹ How highway tunnel ventilation is engineered is beyond what our sources for this section cover.

Why do they keep digging up the same street?

Under a city street sit water mains, sewers, gas lines, and electric and telecom conduits, owned by a mix of public agencies and private companies. Each owner has to dig to fix or replace its own lines, often on its own schedule. (The cables and conduits get their own tour in 311, the wires under the street, and city data.) Two rules try to keep that from wrecking the street and the people digging.

Call before you dig. In Massachusetts, anyone excavating - a utility crew, a contractor or a homeowner setting a fence post - must first premark the area with white paint or stakes, then notify Dig Safe at least 72 business hours ahead. Weekends and holidays do not count. The national call-before-you-dig number, 811, reaches it. Dig Safe is a nonprofit, and it tells the utilities with lines in the area to come and mark them. A ticket is valid for 30 days.⁶⁸

The paint and flags you see on sidewalks are this process made visible: the white premark answers "where will we dig?" and the utilities' marks answer "what is down there?"

Protect new pavement. Many cities restrict digging in streets they have recently rebuilt. In Cambridge, Massachusetts, a street rebuilt or resurfaced within the past five years is a "Moratorium Street" and may not be dug up except in an emergency or with the Commissioner's approval. Every street-opening permit needs a Dig Safe number.⁶⁹ The rules also govern how the hole is closed: the backfill has to be compacted to 95% of a lab standard called modified Proctor density, so the trench does not settle into a dip, and if patches end up less than 10 feet (about 3 meters) apart, the permit holder must mill and repave the street curb to curb.⁶⁹

Moratoriums also work as a coordination tool. A utility that knows a street is about to be repaved has every reason to replace its pipes first, because afterward the street is closed to it for years. Cities use their paving schedules to pull water, gas and electric work into the same window.

In Boston, utilities are expected to finish first: before a street is resurfaced, each utility completes its work and signs off on the block in the City of Boston Utility Coordination Software (COBUCS), the city's reservation system.²⁵⁹

What is Vision Zero?

Vision Zero is a road-safety policy that sets the goal of eliminating all traffic deaths and serious injuries.

The idea comes from Sweden, where Parliament decided by a large majority in autumn 1997 to make Vision Zero the basis of national road safety work.²⁶⁰ In the US, Hoboken, New Jersey, became a Vision Zero city by executive order in 2019. In January 2024 the city reported seven consecutive years without a traffic death and listed measures such as daylighting delineators at 31% of intersections, curb extensions and a 15 mph school-zone limit on 67 blocks.²⁶¹

In Boston, the city joined Vision Zero in 2015 under Mayor Walsh, with a target of zero traffic deaths by 2030. A resolution filed with the City Council on July 22, 2026, recommitting to that goal, cites 11 traffic deaths in 2025, most of them pedestrians. The tools it lists include a lower default speed limit, Neighborhood Slow Streets and a 2023 update to the city's action plan.²⁶²

In our reading, what the policy changes in practice is the default question after a crash: what about the street, and the speed it invited, allowed it to happen, alongside who was at fault. Specific measures such as speed limits and street redesigns are argued over locally, and this guide does not take a side.

What's changing now (2026)

• Bridge inspection: the updated NBIS rule, in effect since June 2022, is the standard states now inspect to, with risk-based intervals from 12 to 48 months.²⁵¹ FHWA last updated its national bridge condition tables in September 2026.²⁵²
• Boston Vision Zero: a resolution recommitting the city to the goal was filed with the City Council in July 2026, with four years left before the 2030 target.²⁶²
• Digging: Massachusetts' 72-business-hour Dig Safe notice and Cambridge's five-year moratorium remain the working rules.⁶⁸⁶⁹

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