Maintaining traffic during construction in a constrained roadway environment is a common practice. Often, work is needed on or near existing lanes of traffic, and the traffic lanes must be moved to perform the needed construction. In many cases, limited room is available to maintain traffic. Shoulders, temporary widening, median crossovers, and temporary alignments are used to avoid road closures, detours, or reducing the number of lanes available to traffic. These temporary conditions can be short-term, intermediate, or long-term durations. Many designers do their best to fit a solution to maintain capacity of a road. The need is more prominent on higher-speed facilities such as freeways and expressways, and it becomes more complicated near entrance and exit ramps. The engineers and designers use available resources such as the MUTCD to introduce lane shifts and closure tapers in the work zone. A few agencies have specified that superelevation (SE) transitioning can use Method 2 SE transitions instead of Method 5 transitions for temporary alignments. Method 2 assumes drivers will accept more discomfort for a temporary construction condition and relies on side friction to the vehicles in curves more so than Method 5 SE transitioning (which is used by most agencies for permanent design). A few other guidelines may be found from agency to agency regarding temporary conditions regarding to permitting narrower lanes in some conditions and some requirements for barrier placement and drop offs near live traffic.
What has been found is that most agencies do not have much guidance available to the engineer regarding temporary geometric design criteria. That often puts engineers in a position to attempt to use design criteria developed for permanent conditions (e.g., those found in the AASHTO Green Book, their State Highway Design Manual, or local standards) or to use a design based on their best judgment or something another agency may have allowed. Guidelines for temporary conditions based on research would be preferred, as this is a reoccurring condition for roads in every jurisdiction. The profession should evaluate what guidelines for temporary conditions should use those specified for permanent conditions and what guidelines may allow for something less. Criteria that need to be examined for temporary conditions include:
· Validating the use of Method 2 SE transitions (including running traffic on lanes with adverse crown – such as an outside shoulder sloping away from the median in a curve to the left).
· Allowable grades.
· Cross-slope breaks (between lanes or even within a driving lane).
· Ramp terminal acceleration and deceleration distances.
· Taper rates.
· Sight distance.
Related questions that could be addressed with temporary traffic control guidelines include:
· Are there any specific locations where traffic control devices must be included or avoided (including regulatory signs)?
· Are engineers using permanent condition criteria when they could be using something less? This could be increasing the cost for the temporary design.
· Are engineers using a design that may not be the safest option in order to fit a solution to the space available?
Having guidelines available for temporary conditions is desirable from an operational standpoint (keeping the traffic moving), from an agency/designer standpoint (reducing their liability by utilizing recognized design research – not under-designing), and a cost standpoint (so temporary conditions are not over-designed).
Designers often overlook drainage impacts of these temporary lane shifts. Often concrete barriers are used to separate the construction work zone from traffic without much consideration of the effect on drainage patterns. Additionally, these lane shifts often do not have shoulders to carry stormwater runoff and therefore localized flooding may occur within the lane transitions. Additional guidance is needed to accommodate drainage considerations for various duration closures, especially those that may be subject to winter driving conditions.
With advances in vehicle and infrastructure technology, a portion of this research should examine what, if anything, needs to be incorporated into the Maintenance of Traffic (MOT) design considerations to aid or facilitate autonomous and connected vehicles through a work zone. What additional traffic control devices may be needed for the vehicle recognition software to recognize the lane transition, especially when horizontal clearances may be limited? The research should evaluate what may need to be incorporated in the MOT design for effective implementation in the field (one possibility raised by the authors of this research needs statement was whether a constant temporary stripe [e.g., 6 inches wide to be recognized by autonomous vehicles] might be needed along each outside edge of a particular direction of traffic).