Large trucks

Most deaths in large truck crashes are passenger vehicle occupants. Large trucks often weigh 20-30 times as much as passenger vehicles. They are taller and have greater ground clearance than cars, which means that lower-riding vehicles can slide beneath truck trailers, with deadly consequences. Strong underride guards can prevent that from happening. 

Truck driver fatigue is a known crash risk. Under federal hours-of-service regulations, drivers of large trucks are allowed to be behind the wheel for as long as 11 hours at a stretch.

Truck braking capability can be a factor in crashes. Compared with passenger vehicles, stopping distances for trucks are much longer, particularly on wet and slippery roads or if the brake systems are poorly maintained.

On average, drivers of large trucks travel many more miles than passenger vehicle drivers, and a larger proportion of those miles are on interstates, which are the safest roads. In 2022, large trucks accounted for 5% of registered vehicles and 10% of vehicle miles traveled (Federal Highway Administration, 2024).

Per unit of travel, large truck drivers were involved in 1.4 fatal crashes per 100 million miles traveled in 2022. This was lower than the rate for passenger vehicle drivers (1.7). Large trucks have a much lower rate per mile traveled of crashes resulting in nonfatal injuries or property damage only compared with passenger vehicles.

In 2022, 4,764 people died in crashes involving large trucks. Seventeen percent of these deaths were truck occupants, 66% were passenger vehicle occupants and 16% were pedestrians, bicyclists or motorcyclists.

The federal commercial vehicle maximum weight standard on the interstate highway system is 80,000 pounds gross vehicle weight, unless a higher maximum weight existed on the state level before July 1, 1956.

Off the interstate highway system, states may set their own commercial vehicle size and weight standards. In most states, the maximum permitted length for a single trailer is 53 feet. Tractors pulling two or three trailers are allowed on some roads; length and weight limits for these longer combination vehicles vary.

Two U.S. Department of Transportation agencies plus individual states oversee large truck safety. The National Highway Traffic Safety Administration (NHTSA) sets standards for new truck equipment. The Federal Motor Carrier Safety Administration (FMCSA) oversees the safety of commercial vehicles in interstate commerce (vehicles operating across state lines) and has some jurisdiction over equipment standards for trucks currently on the road. FMCSA regulations cover equipment, licensing, hours of service and vehicle inspection and maintenance.

States regulate trucks that operate only within their borders, and state personnel conduct roadside inspections of trucks and drivers to enforce federal rules for equipment, hours of service and vehicle maintenance and inspection. Federal and state personnel also conduct reviews of carriers’ compliance with these regulations. Carriers with high rates of crashes and inspection violations are subject to interventions including warning letters, offsite investigations, onsite investigations and suspensions of service. 

Licensing drivers is a state matter, but since 1992, federal law requires states to meet licensing standards for commercial driver’s licenses (CDLs). Prior to 1992, a few states allowed any driver licensed to drive an automobile to drive a large truck or bus, and other states had weak testing and licensing standards for commercial drivers. Since 1992, federal law has established testing, licensing and health standards for issuing CDLs.

Both interstate and intrastate commercial drivers must obtain CDLs if they operate trucks with gross vehicle weight ratings of 26,001 pounds or more, if they transport 16 or more passengers, or if they transport hazardous materials. A national database of all CDL holders helps to deter truckers from obtaining licenses in more than one state and then “spreading” their traffic convictions over more than one license to avoid sanctions for multiple violations. The database also prevents disqualified drivers from being licensed.

If large trucks cross state lines or carry hazardous materials, their drivers must be 21 or older. States can permit drivers ages 18-20 to operate large trucks within the state.

Multiple studies indicate that truck drivers younger than 21 and in their 20s have a higher rate of involvement in both fatal and nonfatal crashes than older drivers (Blower, 1996; Campbell, 1991; Frith, 1994; Teoh et al., 2017). However, at the direction of Congress, FMCSA has launched a pilot program to allow 18-20-year-old drivers to operate large trucks across state lines (Office of the Federal Register, 2022). Young drivers in this program are subject to various safeguards such as strict eligibility requirements, a supervised driving period, and use of crash avoidance technologies. FMCSA plans to compare crash rates of young drivers in the program with a sample of drivers ages 21 and over.

IIHS research has found that truck drivers behind the wheel for more than eight hours are twice as likely to crash (Jones & Stein, 1987). Truckers' long work hours cause sleep deprivation, disruption of normal sleep/rest cycles and fatigue (IIHS, 2000; IIHS, 2008).

Institute researchers found that truck drivers reporting hours-of-service violations are more likely to report having fallen asleep behind the wheel during the past month (Braver et al., 1992;  McCartt et al., 2008). Another study based on a national sample of large truck crashes found that a truck driver's hours-of-service violations or logbook violations increased the likelihood that the truck driver would be found to have precipitated the crash (Blower et al., 2010). The proportion of large truck crashes for which fatigue is a contributing factor is uncertain.

Hours-of-service violations used to be much harder to detect when paper logbooks could be easily falsified. Since 2017, electronic logging devices have been required, making it much more difficult to get around the rules.

Under current FMCSA regulations, interstate commercial truck drivers are not allowed to drive for more than 11 hours or drive after 14 hours since starting a duty shift until they take a 10-hour break. Additionally, drivers are required to spend at least 30 minutes off-duty after no more than eight hours of driving. Drivers can't drive after accruing 60 work hours during a seven-day period or 70 work hours during an eight-day period, but a "restart" provision allows truckers to get back behind the wheel after a 34-hour off-duty period.

Prior to rules implemented in January 2004, truckers were allowed to drive no more than 10 hours without taking an eight-hour off-duty period. There also was no restart provision.

Based on Institute surveys of long-distance truck drivers in Pennsylvania and Oregon, drivers spent more hours behind the wheel after the work rules changed in January 2004 (McCartt et al., 2008).

Drivers also reported more instances of falling asleep at the wheel. In Pennsylvania, 19% of truck drivers admitted to dozing at the wheel at least once during the past month in 2005, up from 13% in 2003, under the old rule. The proportions in Oregon were 21% in 2005 compared with 12% in 2003.

Defective equipment plays a role in many truck crashes.

In the late 1980s, Institute researchers examined crashes of large trucks in the state of Washington and found that tractor-trailers with defective equipment were twice as likely to be in crashes as trucks without defects (Jones & Stein, 1989). Brake defects were most common; they were found in 56% of the tractor-trailers involved in crashes. Steering equipment defects were found in 21% of crash-involved trucks.

Institute researchers conducted a similar study of interstate large trucks in North Carolina during 2010-12 and found that steering equipment defects no longer occurred with enough frequency to analyze (Teoh et al., 2017). Brake defects were found in 42% of crash-involved trucks investigated, and those severe enough to place the truck out of service tripled trucks' crash risk.  Having any vehicle defect also was associated with a 200% increase in crash risk. 

A 2010 study examined the role of defective equipment in crashes included in the Large Truck Crash Causation Study (Blower et al., 2010). The study gathered detailed information on a national sample of 2001-03 crashes involving an evident injury and at least one large truck. Post-crash inspections of the trucks indicated that almost 55% had at least one mechanical violation, and almost 30% had at least one condition serious enough to have taken the vehicle immediately out of service. Of all equipment violations, violations in the braking (36%) and lighting (19%) systems were the most frequent. A truck with an out-of-adjustment brake condition was 1.8 times as likely to be the vehicle that precipitated the crash. In rear-end and crossing-path crashes, brake violations significantly increased the likelihood that the truck was the striking vehicle.

Compared with passenger vehicles, stopping distances for trucks are much longer. On wet and slippery roads, there are even greater disparities between the braking capabilities of large trucks and cars.

Braking disparities can be aggravated by poor maintenance of truck braking systems. New large trucks must have automatic brake adjusters, visible brake adjustment indicators and antilock brakes.

Antilock brakes, which keep wheels from locking during hard braking, improve driver control of trucks during emergency stops and reduce the likelihood of tractor-trailer jackknifing. Antilocks have been required on new tractors since 1997 and on new trailers, single-unit trucks and buses since 1998. They reduce trucks' crash risk by an estimated 65% (Teoh et al., 2017).

In addition to antilock brakes, several other crash avoidance technologies have been developed for large trucks. To date, penetration of these technologies into the large truck fleet has been slow compared with passenger vehicles, but the technologies have the potential to substantially reduce large truck crashes.

Electronic stability control (ESC) has been required on all new truck tractors and buses since Aug. 1, 2019. ESC is designed to intervene when a truck's motion becomes unstable, possibly resulting in rollover, jackknife or other loss of control.

The high centers of gravity on large trucks increase the risk of rolling over, particularly on curved roadway segments, such as ramps. Nearly half of truck occupant deaths each year occur in crashes in which the vehicle rolls over.

Based on an analysis of crashes occurring during 2004-08, the Institute estimates that ESC has the potential to prevent or mitigate as many as 31,000 crashes involving large trucks each year, including up to 20% of moderate-to-serious-injury large truck crashes and 11% of fatal large truck crashes (Jermakian, 2012). NHTSA estimates that ESC on large truck tractors and large buses will prevent 40% to 56% of untripped rollovers and 14% of loss-of-control crashes (Wang, 2011).

The IIHS study also looked at the potential crash reductions with other crash avoidance features — blind spot detection, forward collision warning/mitigation and lane departure warning (Jermakian, 2012). The combination of those technologies and stability control could prevent or mitigate as many as 107,000 police-reported crashes each year, representing 28% of all crashes involving large trucks. As many as 12,000 nonfatal injury large truck crashes and 835 fatal large truck crashes each year could be prevented or mitigated, the study found.

Stability control technology has the greatest potential for preventing or mitigating large truck crashes involving nonfatal or fatal injuries, while blind spot detection has the greatest potential for preventing large truck crashes of any severity.

The actual crash reductions from these crash avoidance systems for large trucks are not yet fully understood, but recent research suggests they have promise. An IIHS study found consistently beneficial effects for stability control, speed governors and electronic devices for logging hours of service (Teoh et al., 2017). Use of other advanced crash avoidance systems were reported too infrequently for analysis.

A recent IIHS study (Teoh, 2021) estimated that forward collision warning and automatic emergency braking systems reduce the rate of front-to-rear crashes per mile traveled of large trucks weighing at least 33,000 pounds by 44% and 41%, respectively. A key benefit, beyond preventing crashes, is mitigating the crashes that do happen. Among front-to-rear crashes of trucks in which these technologies intervened (issued a warning or braked automatically), speed was reduced by about 50%, on average, between intervention and impact.

As part of the bipartisan infrastructure package passed in 2021, Congress ordered regulators to issue a rule requiring AEB on all trucks over 26,000 pounds. Congress also called for a study on whether other types of trucks should be equipped with AEB. In response, NHTSA and FMCSA issued a proposed rule that would require AEB in all new motor vehicles over 10,000 pounds (NHTSA, 2023). This proposal would also extend the ESC requirement to vehicles over 10,000 pounds that were not subject to an existing mandate (generally, those between 10,000 and 26,000 pounds).

While automatic emergency braking offers greater functionality, a benefit of forward collision warning is that some systems can be installed on existing trucks, allowing the benefits to be realized sooner and more broadly.

In an underride crash, a passenger vehicle goes partially or completely under a truck or trailer, increasing the likelihood of death or serious injury to the passenger vehicle occupants. A 1997 Institute study of fatal crashes between large trucks and passenger vehicles estimated that underride occurred in half of these crashes (Braver et al., 1997). Of the underride crashes, 57% involved the front of the truck, 22% involved the rear and 20% the side. 

A federal rule to upgrade the rear-impact guard standard for new trailers took effect in January 1998. Several types of trucks are exempt from the rule, including single-unit trucks, trucks with rear wheels set very close to the back of the trailer and various types of special-purpose trucks. In 2010, the Institute studied how guards built to comply with the federal standards are performing in real-world crashes and found many fail, allowing severe passenger vehicle underride and resulting in serious or fatal injury (Brumbelow & Blanar, 2010).

As a result of this research and some initial crash tests that confirmed the problem, the Institute petitioned the federal government to require stronger rear underride guards on large trucks and improve performance test procedures. Citing this petition, NHTSA issued a proposed upgrade to the rear underride regulations for tractor-trailers in 2015 (Office of the Federal Register, 2015) and finalized the rule in 2022 (Office of the Federal Register, 2022). The final rule largely aligns U.S. regulations with stricter ones in place in Canada since 2007.

In issuing the rule, NHTSA rejected the Institute’s requests to require guard designs with improved performance in offset crashes, to certify guards when attached to a trailer instead of a rigid fixture and to consider reducing the number of trailer types that are exempt from the standard. NHTSA previously decided not to extend underride regulations to single-unit trucks (Office of the Federal Register, 2015).

NHTSA estimated its final rule will have minimal effect because 94% of new trailers already have guards that meet the Canadian standard. Institute crash tests have shown that underride guards built to this standard work well when passenger vehicles strike the center of the trailer’s rear but don’t necessarily prevent underride in crashes involving only a small portion of the truck’s rear. NHTSA recently denied another petition by truck safety advocates asking the agency to require better performance in such crashes (Office of the Federal Register, 2024).

Many trailers already have underride guards that go beyond the federal requirements thanks to the Institute’s own testing program. The IIHS TOUGHGUARD award, introduced in 2017, recognizes rear underride guards that prevent underride in three crash test scenarios — full-width, 50% overlap, and 30% overlap — using a midsize car traveling at 35 mph into the back of a semitrailer. Today, nine North American trailer manufacturers, including the eight largest, produce guards that qualify for the award. Most of the companies have made the improved guards standard on all new trailers.

Stronger underride guard requirements could prevent deaths and injuries in rear- and side-impact crashes.

There is no requirement for front or side underride guards in the United States. European Union regulations have required front underride guards on large trucks since 2003. A 1998 Institute study of fatal truck crashes in Indiana found that 9 out of 44 front underride crashes might have been survivable had underride not occurred (Braver et al., 1998). Side underride guards could be even more effective. A 2012 Institute study found that in the U.S. strong side underride guards have the potential to reduce injury risk in about three-fourths of cases where a passenger vehicle occupant sustained a serious injury from an impact with the side of a large truck (Brumbelow, 2012).

IIHS has conducted two tests of a side underride guard. The AngelWing guard, made by Airflow Deflector Inc., succeeded in blocking a midsize car traveling 35 mph from traveling underneath the trailer (Automotive World, 2017). In a later test, it prevented underride at 40 mph (IIHS, 2017). The AngelWing doesn’t cover the whole length of the trailer, but in combination with the wheels, it provides protection along 62% of it.

In 2023, NHTSA released a preliminary cost-benefit analysis of requiring side underride guards on all new trailers and semitrailers, but its estimate of lives saved were about one-tenth the amount estimated by IIHS (IIHS, 2023). NHTSA’s estimate excluded many types of crashes that are likely relevant such as those involving more than one passenger vehicle. IIHS estimates that a side underride guard mandate could prevent about 160-220 passenger vehicle occupant fatalities each year.