Work Zone - 6.05 Geometric Design & Safety
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6.05 Geometric Design & Safety
Design engineers must design safety into all MDOT work zones and address safety considerations in all MOT’s. Considerations include items such as vertical differentials, vertical / horizontal clearance, lane widths, etc. and must be evaluated when scoping and designing projects.
6.05.01 Lane Transitions and Widths
Existing lane widths should be maintained during construction when feasible. Consider the following factors when narrowing lanes or shifting traffic:
- Overall roadway width available
- Posted speed limit
- Traffic volumes through the project
- Number of lanes
- Length of project
- Duration of the lane width reduction
- Roadway geometry
- Percentage of commercial traffic
- Special vehicles such as campers and boats
Lane transitions, reduced lane width, and traffic control changes in conjunction with the proximity of temporary traffic control devices must be designed in a manner that does not result in an unexpected condition for the motorists.
Projects should have a minimum 11-foot lane, and any shoulder next to an open ditch should be at least 3 feet from the hinge point (2 feet paved with 1 foot aggregate preferred). If the required lane widths and shoulders cannot be maintained during the staging of a 3R / 4R freeway project and the project is next to an open ditch or roadway section, a work zone shoulder width variance request must be completed using Form 5632 Open Ditch 3R/4R Freeway Work Zone Width Variance Request.
Submit variance request to WZMU for review by the Traffic and Safety Statewide Alignment Team (TSSAT). The TSSAT will provide one of the following recommendations.
- The MOT design is acceptable based on the documentation provided and the project can proceed.
- The MOT design is acceptable, but additional mitigation methods, as noted by the review team, should be incorporated into the project, and the project can proceed.
- The MOT design is unacceptable and requires the project office to adjust the MOT design.
Shoulder delineation devices must be used as detailed in WZD-126. Flexible delineators, delineator panels, or flexible guardrail delineators must be used on narrow shoulders. Contact the WZMU to discuss additional mitigation measures.
6.05.02 Shy Distance and Buffer Zones
Shy distance is defined as the distance from the edge of the travel lane to the nearest roadside object or traffic control device.
Lateral buffer zones provide additional space between the motorist and temporary traffic control devices, work zone operations, and equipment and materials. Lateral buffer spaces should be included anytime there is additional space. A 2-foot shy distance is optimal, however, a 1-foot shy distance is acceptable. The TTCP must provide the lateral buffer zone or clearance dimension to be used during construction operations.
Longitudinal buffer zones are provided at the beginning of the work zone between the end of the temporary traffic control transition and the beginning of the active work area. The length of this buffer zone, defined in the maintaining Traffic Typicals.
In locations with intersecting side streets or business driveways, it may be safer to reduce the amount of buffer space below the typical layout to allow for a more defined work zone. If this occurs on a project, documentation explaining the reasons the typical length was reduced should be provided.
6.05.03 Work Zone Clear Zone
The contractor’s operations may be exposed to errant vehicles that enter areas adjacent to the traveled way. A Work Zone Clear Zone (WZCZ) must be maintained by the contractor as part of the WZTCP to address storage of equipment, employee private vehicle parking, storage/stockpiling of materials, etc.
The WZCZ applies during working and non-working hours and will only apply to roadside objects introduced by the contractor’s operations. If the equipment or materials cannot be relocated outside the WZCZ then the items must be delineated or attenuated with appropriate traffic control devices.
The WZCZ and buffer zone dimensions should be evaluated and increased where appropriate for horizontal curves, long downgrades, steep inclines and locations of high traffic volume.
See Section 7.01.10 Clear Zone History of the MDOT Road Design Manual for detailed information on the WZCZ.
6.05.04 Vertical Under Clearance
It is critical to maximize the vertical under clearance on all construction projects. Any reductions to the existing vertical under clearance below the required minimum structure height must be posted on the Active Permit Restriction Bulletin System and signed on the construction project. For more information, see Design Criteria – New and Reconstruction Projects – Vertical Underclearance in Bridge Design Manual.
An orange on black W12-2 sign should be used. The TSC Permit Engineer or Agent can assist with the posting of any reductions to existing under clearance.
6.05.05 Lane Edge
Vertical differentials should be minimized, mitigated, or eliminated. Anticipated vertical differentials should be included in the project plans along with the requirements for warning the motorist. The section titled “Chip Seal Surface Treatment and HMA Construction, Staggered Lane Endings with Vertical Longitudinal Joints,” in section 812 of the MDOT Standard Specifications for Construction provides additional requirements for shoulders and vertical longitudinal joints while maintaining traffic.
Vertical differential treatments are shown in Table 6-8: Lane and Edge Drop Treatments. Uneven lane (W8-11) warning signs should be repeated after each intersection or interchange, and at additional locations as determined by the Engineer.
Table 6-8: Lane and Edge Drop Treatments
| Differential | Distance from the Active Traffic Lane | |
|---|---|---|
| 0 to 3 feet for shoulders or between directional traffic lanes* | 3 to 8 feet | |
| 0 to < 2 inches without taper | Install shoulder drop-off signing or uneven lane signing | Install striped edge line Install Low Shoulder signing (optional) |
| 2 to < 4 inches with 1:3 taper | Install shoulder drop-off signing or uneven lane signing | Install striped edge line Install Low Shoulder signing (optional) |
| 2 to < 4 inches without taper | Install shoulder drop-off signing or uneven lane signing, and install channelizing devices | Install Shoulder Closed signing Install channelizing devices |
| 4 to < 12 inches with 1:4 taper | Install shoulder drop-off signing or uneven lane signing | Install striped edge line Install Low shoulder signing (optional) |
| 4 to < 12 inches with 1:3 taper | Install shoulder drop-off signing or uneven lane signing, and install channelizing devices | Install Shoulder Closed signing Install channelizing devices |
| 4 to < 12 inches without taper | Install shoulder drop-off signing or uneven lane signing, install channelizing devices, and install Type III barricades, at 20 × S spacing | Install Shoulder Closed signing Install channelizing devices Install Type III barricades at 20 × S spacing |
| 12 inches or greater | Install barrier wall or reduce channelizing device spacing** | Install Barrier wall or reduce channelizing device spacing* |
6.05.06 Freeway Ramps
When construction activities require ramp traffic to be maintained off existing alignment, design the temporary ramp alignment to meet a minimum design speed of 10 mph below posted speed prior to construction (Standard Plan R113). Once the proposed ramp work is complete and can be opened to traffic, remove temporary traffic control devices that modified the freeway ramp traffic.
Placing a stop or yield condition for freeway entrance ramps should be avoided by supplying a merge area that is similar to the pre-work zone distance, during the project life. Consider temporarily closing the ramp to complete the necessary work.
If there must be a shortened gap requiring a stop or yield condition in the ramp area, minimize the amount of time the gap is shortened; expedite the work, try to avoid peak traffic times, and restore the ramp to a merge condition as soon as possible.
6.05.07 Relocating Traffic
Project scoping and design staff should verify the existing conditions and thicknesses to ensure the pavement will be able to handle the expected traffic volume (passenger & commercial). Pavement core samples should be taken to verify the condition of the pavement.
A. Road Way
When shifting traffic, review the roadway to ensure the integrity of existing features (i.e. drainage structures, culverts, handholes etc.) can support the expected traffic. The review should take place just before shifting traffic to verify the current pavement condition has not deteriorated or the design has not been modified.
Existing rumble strips should be evaluated for proximity to wheel paths and temporarily filled where appropriate. Determination and guidance is detailed in [[[Work Zone - 6.01 Temporary Traffic Control#6.01.21 Pilot Vehicles|Section 6.01.21]].
B. Vertical and Horizontal Clearance
Vertical and horizontal clearances should be evaluated on shoulders in the same manner as the mainline as detailed in 6.05.04 Vertical Under Clearance.
C. Objects within the Clear Zone
Objects within the clear zone should be identified using object markers. If feasible, object should be removed, otherwise they will need to be protected. Existing guardrail runs should be evaluated with the temporary edge of travel lane and adjustments should be included in the TTCP.
D. Shoulder Conditions
Existing lateral conditions and construction operations may result in a narrow shoulder while maintaining traffic. The project designer should evaluate the cost to pave additional shoulder width in these areas to improve safety and mobility.
All permanent shoulders or shoulder widening that will be used as temporary lanes, with construction speeds of 45 mph or greater, when traffic is shifted onto a shoulder that has been temporarily widened for maintaining traffic must incorporate the Safety Edge per the section titled “Safety Edge” in section 501 of the Standard Specifications for Construction.
When temporarily widening a shoulder for maintaining traffic, the material used must be specified per Frequently Used Special Provision, 20SP307A.
Exhibit 6-33: Safety Edge
INSERT PICTURE
6.05.08 Value Engineering Change Proposals (VECP)
A VECP is a change, proposed by the contractor, to a project’s specifications, plans, or other contract requirements. VECP’s should consider the requirements of the Work Zone Safety and Mobility Policy in the analysis process, see FUSP 20SP-104A. Safety and mobility mitigation costs should be included in the analysis. When reviewing a VECP for approval all factors need to be looked at, including work site safety, which can be difficult to quantify but should be factored into the final discussion process. Simply reducing the lane widths to reduce the project cost should not be considered unless other tangible factors are gained by the width reduction.
It is common for offices to detail a timeframe for review and submittal of VECP’s in the MOT SP. During the VECP process the TMP should be reviewed to determine if the proposed change has already been evaluated during the TMP development.
6.05.09 Transport Permits for Oversize (Weight, Width, Height, & Length) Vehicles
Route availability is important for oversize vehicles. TSC staff must coordinate alternate route availability with the Transport Permit Unit in the Bureau of Development, Development Services Section, when placing restrictions on width, height or weight for oversized vehicles in construction work zones. Special attention “should be given to” routes “to/from” border crossings. Global signing may be utilized depending on the scope scale and complexity of the project. Contact information for Transport Permits may be found on the MiTRIP website.
6.05.10 Temporary Median Crossovers
The use of temporary median crossovers moves traffic away from work areas. Temporary median crossovers should be considered for freeway reconstruction projects and bridge partial/complete replacement projects. The following should also be considered:
- Cost of building and removing crossover
- Construction scheduling compared with the cost of part width construction
- Capacity and delay evaluations.
- On and off ramp locations
- Expedited work schedules
- Roadway geometry
- Drainage
Temporary median crossovers should be designed with the same requirements as the roadway. If permitted, the project manager may complete this design work and build the temporary crossovers as a separate advanced contract.
6.05.11 Traffic Control – Setup, Removal, and Stage Changes
(Updated 12/3/2020)
All Traffic control, including pavement markings, for project set-up and removal must be considered during the development of the TTCP.
A. Traffic Switch
A traffic switch is defined as a change in the present traffic configuration which requires multiple lane lines and/or edge lines to be relocated to a new location and the old lines to be abandoned or removed between construction stages or maintenance of traffic stages. There are times when conflicting pavement markings and sign information is unavoidable, but such occurrences must be minimized by accelerated work. The following factors should be analyzed when considering the time frames for night traffic switch operations:
- Local Crash Histories (with and without the work zone in the area)
- Traffic Speeds
- Traffic Volumes
- CO3 Mobility and Delay Analysis or Synchro Analysis
- Type of Work and Needed Traffic Control
- Duration of Work
- Residential and commercial concerns stated at outreach meetings during planning and design phases.
B. Advance Warning of Traffic Switch Operations
When performing traffic switch operations, it is important to notify the motoring public of changing conditions. When PCMSs are specified in the contract, it is recommended that they are moved in advance of the anticipated queue location. The message on the PCMS should state, Traffic Switch Ahead, and if anticipating traffic backups, a second message added, stating, Be Prepared to Stop, should be placed on PCMS leading into the work zone. If the contract does not have PCMS specified in the contract, rigid static signs stating, New Traffic Pattern Ahead, should be utilized. Whether a traffic switch is performed during the day or night, special consideration needs to be given for motorists in the area of the queue.
C. Law Enforcement During Traffic Switch Operations
See Section 4.05, Work Zone Law Enforcement, for detailed information regarding inclusion of Work Zone Enforcement on MDOT jobs.
Traffic switches performed at night should include the use of Work Zone Enforcement. Frequent coordination with law enforcement posts on this activity is required in order to guarantee police presence at time of traffic switch. Extensions of time will not be approved for delays caused by lack of law enforcement.
6.05.12 Nighttime Traffic Switches
It is recommended that traffic switches are performed during daytime, off-peak hours if feasible when considering the overall project safety and mobility factors. If it is determined that traffic switches must be performed at night, based on the previous factors listed, a Temporary Traffic Plan (TTP) peer review should be conducted and approved by a team consisting of Region, TSC Traffic & Safety, and TSC Construction staff.
While a traffic switch operation is not specifically detailed in the project plans, Designers and Construction Engineers should carefully consider the time frames required for these operations. It is understood that due to mobility requirements, many of these operations should take place within the restrictions noted in the Special Provision for Maintaining Traffic.
Traffic switch operations, however, can be significant, and should be accounted for in the Progress Clause or the Special Provision for Maintaining Traffic. Lane closure restrictions for staged construction could be different than the requirements during traffic switch operations. Therefore, a separate section for traffic switch lane closure restrictions must be clearly defined with what applies during the traffic switch operations.
Construction staff is required to coordinate traffic switch operations with the contractor. It is not uncommon for the traffic switch operations to impact additional roadways or ramps, or take a substantial amount of time during non-peak traffic time frames. Flexibility is encouraged when reviewing and approving the traffic switch time frames, along with other operational logistics.
A. Nighttime Lighting for Traffic Switch Operations
Worker and motorist safety is a shared responsibility between MDOT and the contractor. When working at night, the lighting requirements as detailed in the section titled “Lighting for Night Work” in Section 812 of the Standard Specifications for Construction must be established and deployed for the entire work zone. The appropriate lighting pay items must be included on projects requiring nighttime switches, or other nighttime work
B. Lane Rental or Liquidated Damage Assessments during Traffic Switch Operations
For the traffic switches to have the least impact on safety and mobility, language to address traffic switch operations and how assessments will apply for specific stages should be addressed in the Maintenance of Traffic Special Provision and Progress Clause. Allowable time frames should correspond with off-peak hours, and the contractor should be exempted from lane rental charges or liquidated damages when traffic is switched during the allowable time frames.
Include the following language as part of Progress Clause or Maintenance of Traffic Special Provision: “Traffic switch operations are exempt from rental or liquidated damage assessments for 8 hours for each traffic switch. Traffic switch operations are to take place within the allowable “Work Area Time Restrictions” as shown in the Maintenance of Traffic Special Provision.
