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General
Light weight temporary traffic control devices are not designed to protect workers from errant vehicles, nor do they protect vehicles from hazards often encountered in work zones. Positive protection systems provide greater physical protection for drivers and workers. These systems contain and/or redirect vehicles, thereby preventing intrusion into the workspace. Common positive protection systems contained in this section are temporary barrier rail (TBR), energy absorbing or redirective end treatments, and shadow vehicles with energy absorbing truck or trailer mounted attenuators (TMAs).
Back to topPositive Protection Use
Positive protection use is outlined in §630.1108(a) of 23 CFR 630 Subpart K. The use of positive protection should be based on engineering judgment. If used improperly, a positive protection system can be hazardous for roadway users. A positive protection system should only be used when the severity of hitting it is less than the severity of impacting the hazard it is designed to protect. Each positive protection system must be carefully analyzed to ensure its use is appropriate for the specific situation. If possible, the hazard should be eliminated or moved beyond the clear zone.
Consider the following mitigations before using positive protection:
Reroute traffic and close the road.
Use an onsite detour to offset traffic from the work area.
Limit the depth of excavations during nonworking hours.
Provide additional buffer between traffic and work activity.
Common situations where positive protection may be warranted include:
Work in locations that provide workers no means of escape from an errant vehicle, such as:
Bridge deck overlays/reconstruction.
Bridge rail construction/retrofit.
Bridge inspections.
Work in tunnels or under a bridge.
Work next to permanent median barrier.
When workers use lifts or buckets to perform overhead work.
Work operations that place workers close to open lanes of traffic for more than two weeks.
Roadside hazards left in place overnight or longer such as:
Drop-offs on structures.
Drop-offs or excavations near the traveled way (refer to Section 9C-2).
Excavations for bridge piers.
Fixed objects that cannot be easily removed, including temporary supports for structures.
Work zones in place for one or more construction seasons with high traffic volumes and high operating speeds.
Separation of two way traffic in freeway work zones.
- Pedestrian protection in work zones.
Temporary Barrier Rail (TBR)
TBR is a safety device designed to prevent errant vehicles from penetrating into areas behind the barrier. A TBR installation consists of individual sections of TBR connected end-to-end. Its primary purposes are:
protection of drivers from obstacles in the work area,
protection of workers from vehicles leaving the traveled way, and
separation of high speed, multilane traffic flowing in opposite directions.
Iowa DOT uses two types of TBR: concrete (BA-401) and steel (560-7). Concrete TBR is mostly used on roadway projects; however, it is also used on many bridge projects where lane width is not an issue. Steel TBR is used almost exclusively on bridge projects where a narrow width barrier is required. Contact the Roadside Safety prior to using steel TBR.
Other proprietary, crashworthy barrier systems are available from various suppliers. These systems each have differing performance characteristics, which must be considered prior to use on a project. These barrier systems should be considered for special circumstances such as:
- narrow width considerations,
- ease of installation and removal, and
- frequent relocation to handle directional peak hour traffic.
Although these systems will likely be more expensive than traditional barriers used in Iowa, their use may offset the cost by providing other benefits for safety and mobility.
Back to topLane Width
A TBR installation must be designed to provide adequate lane width for vehicles to travel through the work zone, yet at the same time provide sufficient space for construction activity. Sometimes compromises must be made to achieve acceptable results.
While it is desirable to maintain full width shoulders adjacent to a barrier, it is usually not practical. Strive to provide at least a 2 foot shy distance from the barrier. For barrier that is to be left in place over winter, the distance is increased to 3 or 4 feet to allow for snow removal operations.
The further the barrier can be placed from the traffic lane, the less likely it will be hit by vehicles. However, barrier offsets between 4 and 8 feet can cause operational problems when motorists attempt to use the narrow shoulder in an emergency. There is not enough room for the vehicle to safely vacate the lane. This results in reduced capacity when other motorists change lanes to avoid the vehicle that is still partially in the lane. It also increases the likelihood of a crash.
Multilane Traffic in a Chute
Staging often necessitates placing traffic between two lines of barrier, commonly referred to as a chute. Crashes that occur in the chute can lead to additional congestion. Designing the chute properly to mitigate operational problems due to a crash is important.
On multilane roadways, the minimum width between barriers should be 30 feet for two lane freeways or expressways. This dimension provides room for two 12-foot lanes and 3-foot shoulders on each side. This width also helps to maintain the roadway speed and traffic capacity.
Cross sections less than 30 feet wide for distances more than 1000 feet should be avoided since this will result in reduced traffic capacity and increase the likelihood of crashes. A reduced regulatory speed limit should be considered.
A minimum width of 26 feet is allowed for short lengths (up to 1000 feet). Due to the narrow width, an advisory speed or a reduced regulatory speed limit is warranted.
Single Lane Situations
Maintain a minimum width of 14 feet 6 inches between TBR and bridge rail or other barriers whenever possible. This provides sufficient clearance for legal width loads and a 1-foot cushion. Loads wider than 13 feet 5 inches require a special permit. See the Iowa Truck Information Guide (https://iowadot.gov/mvd/motorcarriers/truckguide.pdf) for more information on permitting requirements.
| Whenever a 15 foot 6 inch clearance cannot be provided, special signing must be placed in advance of the work zone. Notify the Traffic Control Engineer and include Standard Road Plans TC-81 and SI-881 or SI-882, as appropriate, in the project plans. |
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| If a TBR installation results in a lane width less than 10 feet 6 inches, contact the Traffic Control Engineer for a TBR design review. |
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Access to the Work Area
An important consideration of any TBR installation is the impact it has on the flow of construction vehicles. Provide an entrance to the work area that is wide enough to allow passage of the various types of construction vehicles that will be present on the project. However, to discourage the general public from inadvertently entering the work area, the entrance should be as narrow as possible and should be offset from the flow of traffic. The use of adjacent shoulders as part of the entrance is encouraged.
Wherever space and work zone configuration allow, provide a separate exit from the work area for construction vehicles. This prevents exiting vehicles from using the entrance, which could cause bottlenecks or backups onto the open traffic lane.
Placement and Anchoring
Whenever possible, TBR should be installed at a minimum of 2 feet from the edge of the nearest traffic lane. This distance provides drivers with a level of comfort and maintains the flow of traffic through the work zone.
The distance TBR is installed in front of a hazard is also a significant design consideration. Although they are very heavy, TBR may slide several feet when impacted by a vehicle. Therefore, sufficient clear distance must be provided behind the TBR to allow for this movement. It is also necessary to provide sufficient work room behind the barrier for contractor equipment to operate. A two-foot clearance behind the barrier should be considered a minimum for contractor operations.
Anchoring TBR to the pavement reduces the amount of clear distance required behind the TBR but increases costs and can damage pavement. Therefore, avoid placing TBR in locations requiring anchoring whenever possible. Anchorage requirements and minimum offsets are shown in Table A of Standard Road Plan BA-401 and Road Design Detail 560-7.
Back to topAlignment
TBR generally follows a constant offset from the traveled lane. However, it is often necessary to transition a line of TBR toward traffic, especially at the beginning of the project or near entrance ramps. The maximum flare rate toward traffic is 6:1, measured parallel to the path of adjacent traffic. Due to the limited space between individual sections of TBR, the 6:1 flare rate must be developed incrementally using a minimum of four TBR sections. Figure 1 illustrates how the 6:1 flare rate is developed.
Figure 1: Concrete TBR Transition to 6:1 flare.
Back to topEnd Treatments
The end of the TBR must be protected if it terminates within the clear zone distance of the temporary traffic control zone (see Section 8A-2) for approaching traffic.
Consider these options for end treatment:
Flare the TBR beyond the temporary traffic control clear zone distance.
Where posted speed limit is 35 mph or less, the tapered end section on BA-104 is considered adequate end treatment for concrete TBR.
Protect the approach end of TBR with a crash cushion. See Section 8C-5 for specific requirements and limitations for crash cushions.
On high-speed roadways, offset of the barrier end treatment a minimum of 5 feet from the edge of the traveled way.
Truck or Trailor Mounted Attenuators
A truck or trailer mounted attenuator (TMA) is an energy absorption device that can serve as a temporary barrier when placed between live traffic and a work area on highways that must remain open to traffic during repairs or accidents. The TMA may significantly help to minimize injuries or fatalities associated with a collision between a vehicle and a truck used for protecting (blocking) a work area.
TMAs should be used to protect people, equipment, and or materials in a work or accident area that is part of a closed traffic lane and/or shoulder while the road remains open to traffic. They are especially useful for short term operations when positive protection is needed but installing fixed barriers may not be possible (e.g. short-term bridge work). TMAs can be used for both stationary and moving operations. Diagrams showing the proper use of TMAs can be found in the Manual of Uniform Traffic Control Devices (MUTCD), Part 6.
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