EPA’s summary table of exhaust standards for nonroad diesel engines has columns for non-methane hydrocarbons, nitrogen oxides, particulate matter, carbon monoxide and smoke opacity, plus useful life and warranty periods. It has no column for carbon dioxide, and there is no footnote explaining the absence, because none is needed. The federal engine program was built to address air quality and public health, and greenhouse gases were never inside its terms.
That distinction is usually lost in the phrase “construction emissions”, and losing it produces confused specifications. Carbon dioxide from an excavator tracks the diesel it burns and nothing in the tier structure limits it. What the tiers changed, by more than an order of magnitude, is what comes out alongside it.
What the tiers actually tightened
Take one power band and read across the four tiers. The band from 130 to 225 kilowatts covers most excavators, dozers and motor graders on a highway job. Its Tier 1 standard, applying to model years 1996 through 2002, allowed 9.2 grams of nitrogen oxides and 0.54 grams of particulate matter per kilowatt-hour. The Tier 4 standard applying from model year 2014 allows 0.40 grams of nitrogen oxides and 0.02 grams of particulate matter. Those are reductions of about 96 percent in each pollutant, achieved across roughly two decades.
The fuel side moved with the engines. EPA states that it reduced in-use diesel sulfur by more than 99 percent, giving ultra low sulfur diesel with a maximum sulfur concentration of 15 parts per million, on the stated reasoning that emission control devices can be damaged by sulfur. That dependency is why the measure was written as a combined engine and fuel program rather than as an engine rule, and it is the clearest case in the field of a hardware standard being unreachable until somebody changed what came out of the pump.
The sequence of rules is on record: Tier 1 for engines at or above 37 kilowatts came in a June 1994 final rule, Tiers 2 and 3 in October 1998, and Tier 4 in the June 2004 nonroad diesel rule. The standards themselves live in 40 CFR Part 1039, with test procedures in Part 1065 and compliance provisions in Part 1068.
The largest engines got the loosest number
Read down the table rather than across it and a discontinuity appears that rarely features in discussions of Tier 4. Engines between 560 and 900 kilowatts and those above 900 carry a Tier 4 nitrogen oxides standard of 3.5 grams per kilowatt-hour from model year 2015, against 0.40 grams for the 130 to 225 kilowatt band. The largest machines are held to a limit nearly nine times higher. Their particulate standard, 0.04 grams per kilowatt-hour, is twice the mid-range limit.
There is a further carve-out inside that band: the table’s notes set the nitrogen oxides standard for generator sets at 0.67 grams per kilowatt-hour and their particulate standard at 0.03. Stationary power on a job site is therefore regulated more tightly than the largest mobile equipment.
For a highway project this matters in proportion to how much mass has to move. A job dominated by large scrapers and the biggest class of dozers is running the least tightly regulated equipment on the site, and the volumes that put them there are fixed by the alignment long before procurement, as earthwork and grading describes.
A tier is a model year, not a machine
A tier label is not a hard cutoff. Certain standards are marked as phase-out standards, and not more than 50 percent of a manufacturer’s production may meet them in each year of the phase-out period. Others are phase-in standards, and at least 50 percent of production must meet them during each year of the phase-in. For several model years, in other words, the fleet leaving the factory was a deliberate mixture.
A second provision does similar quiet work: useful life. For engines at or above 37 kilowatts EPA defines useful life as 8,000 hours or ten years, whichever comes first, with an emission warranty of 3,000 hours or five years. Those are regulatory definitions rather than predictions, and construction equipment is routinely kept far longer than either. A grader bought in 2006 to a Tier 3 standard can still be earning on a project today, entirely legally, with no federal obligation to improve.
Which is why the federal program alone cannot decarbonize or clean up a construction site in any given year. It cleans up the machines being sold.
Reaching the engines already on site
The instrument that reaches existing equipment is state regulation, and the developed example is California’s. The Air Resources Board’s regulation for in-use off-road diesel-fueled fleets applies to all self-propelled off-road diesel vehicles of 25 horsepower or greater used in California, including rented and leased fleets. Its current form was adopted in November 2022, was approved and filed with the Office of Administrative Law on August 18, 2023, took effect on October 1, 2023, began imposing requirements on January 1, 2024, and received EPA authorization on January 10, 2025.
Its mechanisms are worth distinguishing. Fleets must meet a fleet average requirement or demonstrate best available control technology. A phase-out of the oldest engines began on January 1, 2024. Reductions can be achieved by retiring, replacing or repowering vehicles, or by installing verified diesel emission control strategies. Medium and large fleets are restricted from adding Tier 4 interim vehicles, which closes off the option of complying by buying somebody else’s transitional equipment. Vehicles are reported to the state and labeled, and each fleet files an annual affirmation each March.
CARB reports that since 2007 the sector has achieved roughly a 17 percent reduction in oxides of nitrogen and a 21 percent reduction in fine particulate matter in California. Two decades of regulation on a slowly turning capital stock produced changes of that order, which is a useful calibration against the claims made for any single equipment measure anywhere in sustainable infrastructure coverage.
Five minutes, and the exemptions a paving crew lives inside
The same California regulation carries an idling limit, and it is the one requirement in this field that an agency can see from the side of the road. No vehicle or engine subject to the regulation may idle for more than five consecutive minutes, a limit CARB describes as effective and enforceable from June 15, 2008. All medium and large fleets have been required since March 1, 2009 to hold a written idling policy informing operators of the limit and to make it available to them. Responsibility for a violation rests with the vehicle owner regardless of who was operating, except for rented or leased equipment, where it passes to the renter.
The exemptions are where the rule meets the work. Idling is permitted when necessary to complete work the vehicle was designed for, and CARB’s own examples are concrete agitation and hydraulic power to a crane or a fuel pump. It is permitted for servicing, repair, testing and maintenance, and a footnote extends that to the periodic idling required to regenerate an exhaust filter. It is permitted where needed for safe operation. A fleet owner may also apply for a waiver for additional idling on justification.
Set that list against a paving operation and much of the idling on a job site is exempt by design. A mixer that stops agitating ruins its load. The measure is real, and its reach is narrower than the five-minute headline suggests.
Zero-emission equipment is currently a queue
California funds zero-emission off-road equipment through the Clean Off-Road Equipment Voucher Incentive Project, which offers vouchers toward the purchase or lease of commercially available zero-emission equipment across categories including construction and earthmoving, and also funds charging and fueling infrastructure. CARB states that the project has been oversubscribed in every funding cycle since it first opened for voucher requests in 2020, and that it is scheduled to open for new requests in the third quarter of 2026. It exists because of what CARB calls the higher upfront cost of zero-emission technology.
Demand exceeding supply of public money in every cycle is evidence that buyers want the equipment. It is not evidence of how much equipment is working, and this publication has not found a deployment count in CARB’s own material for that program. An agency wanting current figures should ask CARB directly, and should ask separately for vouchers issued and equipment delivered, since a queue is not a fleet.
Renewable diesel changes the fuel, not the fleet
The California regulation also requires fleets subject to it to procure and use renewable diesel, specified as R99 or R100, in all vehicles they own or operate in the state from January 1, 2024, with limited exceptions including lack of availability. This is the one measure in the set that reduces carbon without touching the machine, since it is a drop-in replacement in existing engines.
How much carbon it reduces depends entirely on the feedstock and the production pathway, and none of the material reviewed here quantifies it. The authoritative source for those figures is CARB’s own fuel pathway certification under the Low Carbon Fuel Standard, which publishes carbon intensity values by pathway, and any project-level claim should cite the specific certified pathway for the fuel actually delivered rather than a generic figure for renewable diesel.
What is left inside the contract
Sort the levers by who controls them and the picture for a highway agency is clear enough. The engine standard is set federally and applies at manufacture. Fuel sulfur is set federally. Renewable diesel content and fleet turnover are set by state rule where such a rule exists, and by the contractor’s capital decisions where it does not. Zero-emission equipment depends on an incentive queue.
What remains inside an agency’s own contract is hours of operation and the sequence that produces them. Fewer passes, shorter hauls, less standing time, fewer nights of double-shifting, and a schedule that does not idle a crew waiting on a delivery. The consequences of that scheduling choice reach past emissions, as night highway construction sets out, and the construction stage’s place in the wider accounting is covered in how highway projects measure embodied carbon. That is why construction-stage carbon in FHWA’s pavement assessment tool is calculated as equipment type multiplied by equipment hours, drawn from EPA’s MOVES model, and why the estimate moves when the schedule moves rather than when the fleet does.