Developmental History of "Highly Mobile Barriers"
Mobile Barriers MBT-1® is what is known as a “highly mobile barrier’ which FHWA has listed under “longitudinal barriers” (B-178). Mobile Barriers MBT-1® was the first and only such barrier to be tested & FHWA accepted under NCHRP 350 & MASH for TL-2 and TL-3 use on the National Highway System (NHS).
Prior to the Mobile Barriers MBT-1® , what was classified as “movable” barrier typically moved laterally within a lane and was used for long lengths or long duration work. Until the MBT-1®, what was classified as "highly mobile barrier" were prototypes that never achieved FHWA acceptance. In contrast, Mobile Barriers MBT-1® achieved FHWA acceptance and has helped revolutionize work zone safety by enabling teams to rapidly setup/remove work zones and quickly place MASH TL-3 positive protection between employees & live traffic.
Crash Test Standards Overview
MASH certified roadside equipment and barriers provide state-of-the-art safety for road users.
"MASH compliance could be the difference between life and death during a high-speed crash which is why it is crucial for every road system" explains another safety device manufacturer.[5]
Without speed-appropriate barriers between live traffic and work zones (i.e. Positive Protection), both drivers & roadside workers face increased risk of serious injury or death from errant drivers. The benefits of complying with MASH include:
- Helping save the lives of workers
- Helping save the lives of drivers & passengers
What standards are applicable to highly-mobile barrier systems?
Applicable test requirements for highly-mobile barrier systems are detailed in a 2004 TTI report :
"Actual testing of the highly-mobile work zone barrier shall be in compliance with NCHRP Report 350 (5), or the current equivalent of that document. As discussed in the interim report, Test 3-11 is considered a structural adequacy test for the barrier system. The impact conditions consist of a 2000-kg pickup truck impacting a barrier at 100 km/h (62 mph) and 25 degrees, with initial contact occurring at the critical impact point (CIP). The objective of this test is containment and redirection of the vehicle in a stable manner (i.e., without overturning). The CIP is selected such that the impact loads in the barrier members are maximized, thus maximizing the structural failure of the rail. This test controls the size and strength of the barrier members, and dictates the dynamic design deflection of the barrier system. While Test 3-10, 820 kg passenger car impacting at 20 degrees and 100 km/h, would typically be required, it may be waived if good anticipated performance can be predicted by other means. This would include previous crash tests on similar member types and geometries. The main concern with this test is snagging potential that would cause vehicle instabilities or the possibility of high occupant risk values. The outcome of the tests should be evaluated per the compliance standard values for longitudinal barrier (i.e., vehicle stability, occupant risk values, occupant compartment deformation, etc.). In addition, damage to the barrier and level of intrusion are critical to the good performance of the highly-mobile barrier system.
It will likely be desirable for the mobile protection system to provide rear as well as longitudinal protection. The accommodation of rear-end impacts is generally accomplished through some form of crash cushion. For mobile operations, the crash cushion typically takes the form of a TMA. This can be accomplished by having a shadow vehicle (commonly a dump truck) equipped with a TMA follow the longitudinal mobile protection barrier system. In this configuration, an approved (i.e., NCHRP Report 350 compliant) TMA would be used on a vehicle of appropriate size and mass, and no further testing would be required."
What are Crash Test Levels?
MASH is the new criteria for crashworthiness of safety devices on the National Highway System (NHS) & supersedes NCHRP 350. The crash test levels (i.e. TL-2, TL-3) specified by the crash test standards are used to test & categorize devices for different anticipated traffic speeds and traffic characteristics.
Crash test levels have become more severe to reflect modern, larger vehicles impacting roadside hardware with greater kinetic energy resulting in greater crash severity (reflected in the requirement for "Impact Severity"). The MASH crash test levels use larger, heavier, more modern vehicles compared to prior NCHRP 350 test levels. Since the NCHRP 350 standard was released in 1993, the average U.S. passenger vehicle has grown 4 inches wider, 10 inches longer, 8 inches taller, and 1,000 pounds heavier over the past 30 years, according to a recent Insurance Institute for Highway Safety (“IIHS”) study.
Each device which has been tested & accepted by the FHWA for use on the National Highway System (NHS) receives an eligibility letter. The eligibility letter specifies the speed appropriate conditions for which the device was designed, tested and rated (eg. TL-2 or TL-3), and whether that is under the new more rigorous MASH test standards, or old NCHRP 350 standards being phased out. For the safety of workers and the public, be sure the safety device is speed appropriate for traffic in the area (typically TL-2 for low speed conditions, and TL-3 for higher speed roadways). [6] For example, many devices have received FHWA acceptance for use in slower traffic areas, but not along highways.
- TL-2 devices are designed and tested for impacts not exceeding 43.5 mph (70 km/h).
- TL-3 devices are designed and tested for higher speeds and generally considered more speed appropriate for interstate and other high speed uses.
MASH
According to a recent Insurance Institute for Highway Safety (“IIHS”) study, the average U.S. passenger vehicle has grown 4 inches wider, 10 inches longer, 8 inches taller, and 1,000 pounds heavier over the past 30 years. Significantly, the IIHS study found that vehicles with higher front ends (pickup trucks, SUVs and vans with a hood height greater than 40 inches) were 44% to 45% more likely to cause fatalities in crashes with pedestrians than smaller cars and trucks.
MASH is the new criteria for crashworthiness of safety devices on the National Highway System & supersedes NCHRP 350. Released in 2016, MASH updates NCHRP 350 in several critical areas to improve safety for road users & reflect modern, heavier, and higher center-of-gravity (C.G.) vehicles. Crash performance is judged on structural adequacy, occupant risk, and vehicle trajectory. Several notable changes include:
- Test vehicle weight requirement increased by 600 lbf to 5,000 lbf
- Test vehicle center-of-gravity (C.G.) requirement increased by 2.4 inches to ≥28 inch high
- TL-2 Impact Severity requirement increased 13.5% to 76.6 kJ
- TL-3 Impact Severity requirement increased 13.5% to 156.4 kJ
NCHRP 350
Released in 1993, NCHRP 350 brought updates that reflected vehicles of the early 1990s. For the first time, NCHRP 350 also introduced crash test levels which differed by intended speed (i.e. TL-2: 70 km/hr or 43.5 mph, TL-3: 100 km/hr or 62 mph). These crash test levels were intended for most service requirements. NCHRP 350 also brought greater uniformity to regulations & testing for both permanent and temporary roadside safety features (including devices such as longitudinal barriers, transitions, end terminals, crash cushions, breakaway/yielding supports, truck mounted attenuators and work zone traffic control devices).
The Federal Highway Administration (FHWA) sunsetted NCHRP 350 criteria on December 31, 2019 and urged agencies to replace/upgrade existing hardware to comply with MASH.
Table 1. Differences between MASH & NCHRP 350 Criteria for Longitudinal Barriers.
| Topic | MASH | NCHRP 350 (Sunsetted) |
|---|---|---|
| Test Vehicle ("Pickup Truck") |
Visual comparison of MASH pickup truck (left) & NCHRP 350 pickup truck (right). |
|
| Test Vehicle (Pickup Truck) | 2270P (5,000 lbf): 600 lbf increase, higher C.G. |
2000P (4,400 lbf) |
| Mass (inertial & gross static): | 2270 kg +/- 45 kg (5,000 lbf ± 100 lbf) | 2000 kg +/- 45 kg (4,400 lbf ± 100 lbf) |
| C.G. height: | ≥28 inch Minimum | 70cm +/- 5cm (25.6-29.5 inch) Recommended |
| Model year: | Within 6 years of the year of test | Within 6 years of the year of test |
| TL-3 (Test 3-11) Test Conditions for Longitudinal Barriers | 156.4kJ nominal impact severity (13.5% increase), | 138.1kJ nominal impact severity, |
| Impact Severity: | ≥144 kJ Required | 127.3-149.4 kJ Recommended |
| Test Vehicle: | 2270P (5,000 lbf) | 2000P (4,400 lbf) |
| Speed: | 100 km/hr ± 4 km/hr (62 mph ± 2.5mph) | 100 km/hr ± 4 km/hr (62 mph ± 2.5mph) |
| Angle: | 25° ± 1.5° | 25° ± 1.5° |
| TL-2 (Test 2-11) Test Conditions for Longitudinal Barriers | 76.6kJ nominal impact severity (13.5% increase), | 67.6kJ nominal impact severity, |
| Impact Severity: | ≥70.5 kJ Required | 60.1-75.5 kJ Recommended |
| Test Vehicle: | 2270P (5,000 lbf) | 2000P (4,400 lbf) |
| Speed: | 70 km/hr ± 4 km/hr (43.5 mph ± 2.5mph) | 70 km/hr ± 4 km/hr (43.5 mph ± 2.5mph) |
| Angle: | 25° ± 1.5° | 25° ± 1.5° |
| Evaluation Criteria Changes [4] |
|
History of "Highly Mobile Barriers"
TTI Mobile Work Zone Barrier (c. 1982)
Station wagons with guardrail. Not commercialized.
TTI Mobile Work Zone Barrier (c. 1984)
Dump trucks with beam. Not commercialized.
Caltrans Balsi Beam (c. 2002)
Limited to stationary applications where the potential for impact will not exceed 43 mph. Not for use in slow moving operations. Rotating beams require on-site setup on arrival and departure to deploy jack stands, stabilize and rotate. Many moving parts. 20-30’ work area, 3’ high, 42-52’ OAL. No integral work lights, power, signage or storage. Limited carry capacity. Sensitive to uneven weight distribution. Internally tested to reported TL-2 levels under NCHRP 350. Not independently tested. No FHWA acceptance/eligibility letter. Only a few made. Limited use in California. Not used outside of California. Not commercially available. 2 Patents.
Mobile Barriers MBT-1 (c. 2008)
Can be used for stationary or slow moving operations. Speed appropriate for interstate use. No on-site setup. Simply drive in place and work. Virtually no moving parts. 20-80’ work area, 5’ high, 42-102’ OAL. Integral lights, power, signage and storage. Carries loads in and on decks up to 85,000 GVWR. Independently tested to TL-3 levels under both NCHRP 350 and the new MASH standards. FHWA acceptance/eligibility letter. Deployed internationally. Recipient of numerous awards. Some have been hit multiple times, even by semis, and are still in service. Commercially available for sale, lease or rent. Various accessories available. Special versions for fire/incident management, security/defense, and international markets. 20+ Patents.
The FHWA has clarified that Mobile Barriers MBT-1® qualifies for various types of federal funding. Mobile Barriers MBT-1® was tested and FHWA accepted under NCHRP 350 & MASH for use on the National Highway System (NHS). MBT-1® was accepted both for TL-2 use in low speed situations, where for example, vehicular traffic is moving at speeds of 43 mph (70 km/hr) or less, and for the even more challenging TL-3 use, such as for the much higher speeds typically encountered on interstates and in many situations.
From the crash video and report, it can be seen that MBT-1® not only met the NCHRP 350 & MASH criteria for high speed impacts - it did so very well. The crash vehicle’s occupant risk values were only a fraction of the allowable limits and there was no damage or intrusions into the passenger compartment.
"[The barrier itself] “did not experience any structural damage. The wall plates were not torn, punctured, or gouged. The test vehicle did not snag the wall panels and the vehicle impact did not create any snag points for future vehicle impacts. The only evident damage to the wall plates was aesthetic, where the test vehicle’s paint and tires made marks on the barrier. The wheels scratched the barrier, but the wall panels remained smooth.”
Table 2. Comparison of MBT-1 to Balsi Beam.
Mobile Barriers MBT-1 |
Balsi Beam |
|
|---|---|---|
|
|
 |
|
| Crash Test/Usage | ||
| FHWA Acceptance/Eligibility Letter | ✓ Yes
FHWA Safety Eligibility Letter B-178 |
✘ No |
| Independent Test & FHWA Review | ✓ Yes | ✘ No |
| Impact Severity Tested | 150 kJ | 22 & 66 kJ |
| Speed Appropriate | ✓ MASH TL-2 & TL-3 (for up to & over 43 mph)
* Updated test vehicle to reflect modern, heavier, and higher C.G. vehicles (2002 pickup, 5135lbs, C.G. height of 29.1 inches). |
⚠ NCHRP 350 TL-2 only (for under 43 mph)
* Old, lighter weight, & lower center of mass test vehicle (1994 pickup, 4325 lbs, unknown C.G. height). * Destroyed a Balsi Beam in failed higher level test. * Limited to locations where "the potential for vehicle impacts will not exceed [NCHRP 350] Test Level 2 conditions (i.e. under 70 km/h (43 mph))." [3] |
| Rear Impact Appropriate | ✓ Yes Integrated TMA (TL-2 or TL-3) |
⚠ Potential Collapse "It is also recommended that the PMWB [Balsi Beam] be outfitted with a truck-mounted attenuator (TMA) and followed with a shadow vehicle that also has a TMA. This is recommended because no testing was done on the PMWB [Balsi Beam] to ensure that a rear hit on a deployed system will not collapse the system". [3] |
| Snagging & Debris Hazard | ✓ None 
"The test vehicle did not snag the wall panels and the vehicle impact did not create any snag points for future vehicle impacts. The only evident damage to the wall plates was aesthetic, where the test vehicle’s paint and tires made marks on the barrier. The wheels scratched the barrier, but the wall panels remained smooth.'” |
⚠ Snagging & Debris 
"Each of the two [NCHRP TL-2] tests conducted [on the Balsi Beam] involved significant sheet metal damage [to the impacting vehicle]... A large section of sheet metal from the vehicle was wedged in-between the overlap in the box beams [of the Balsi Beam]... [In test number 642,] the right front corner of the [pickup truck] vehicle sustained significant damage due to the impact with the box beam overlap. Only the lower control arm was still attached to the front wheel hub assembly. All of the sheet metal that covered the right corner of the [pickup truck] vehicle was torn away. The battery was exposed and only supported by its wiring... The hood of the [older, lower center of gravity pickup truck] vehicle overlapped [penetrated] the front face of the barrier 387 mm (15.4 in) [& debris from the pickup truck was wedged on top of the barrier over nearly the full width of the box beams]" [3] |
| Structural Damage |
✓ None 
"[The MBT-1® barrier itself] 'did not experience any structural damage. The wall plates were not torn, punctured, or gouged." |
⚠ Scraping & Gouging 
"[The Balsi Beam itself did experience] small to moderate amounts of scraping and gouging of the barrier’s box beams. Varying amounts of vehicle sheet metal will accumulate at the overlapping joint of the box beams and will have to be removed before the barrier can be redeployed." [3] |
| Barrier Height |
✓ Shoulder High
* 60 inches (5ft) * Visual barrier - traffic can't see in, crews can see out. * Redirected semi-trucks on 2 seperate occasions. |
⚠ Waist High 
* 36 inches (3ft) * "The hood of the [older, lighter weight, & lower center of gravity pickup truck] vehicle overlapped [penetrated] the front face of the barrier 387 mm (15.4 in) [& debris from the pickup truck was wedged on top of the barrier over nearly the full width of the box beams]" [3] * Semi-truck impact would be higher than Balsi Beam wall. |
| Skidding | ✓ None |
⚠ Skidding 
"Both the tractor truck and trailer [28,000 lbf Balsi Beam] were pushed away from the impact area [skidded laterally by at least 3.0 inches into the work zone, during the impact with the older, lighter weight, & lower center of gravity pickup truck]". [3]
|
| Barrier | ||
| Work Area | 20-80' | 20-30' |
| OAL | 40-100' | 40-50' |
| Barrier Height | 60 inches (5ft) | 36 inches (3ft) |
| Weight (approx.) | 50,000 lbs | 28,000 lbs
* "Both the tractor truck and trailer [28,000 lbf Balsi Beam] were pushed away from the impact area [skidded laterally by at least 3.0 inches into the work zone, during the impact with the older, lighter weight, & lower center of gravity pickup truck]". [3] |
| GVWR (max) | 85,000 lbf | 60,000 lbf |
| Tractor | Standard Tractor | Specially Modified Tractor Required |
| Setup (On Road) | No Setup (simply stop & work) | On-Road Setup Required |
| Moving Operations | Fully Mobile | Immobile (tractor locked/jacks down)
* If moved, jacks must be raised, it must move very slowly and straight forward, or tractor also unlocked. It has not been tested, even at TL-2, with the tractor unlocked and jacks raised. |
| Features/Options | 20' Decks & Lockable Storage, Onboard Power, Air & Lights, Signage & TMA. Can carry materials, supplies, crane and other equipment. Load does not have to be centered. Full lane access w/ as little as 6" incursion. |