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From page 19... ... 17 3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS This chapter catalogs design treatments that can be considered for use in addressing nonrecurrent congestion. A detailed summary of each of the design treatments is provided that includes the following information: • Description and objective • Typical applications • Design criteria • How treatment reduces nonrecurrent congestion • Factors influencing treatment effectiveness • Cost The design treatments are classified on the basis of similarities with respect to function or location on the roadway. Read the entire page →
From page 20... ... 18 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Temporary construction crossovers (see discussion under Contraflow Lanes for Work Zones) • Median openings controlled by manual or automated gates (see discussion under Gated Median Barriers) Read the entire page →
From page 21... ... 19 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Although emergency crossovers are typically provided on freeways, median openings on arterial roadways can also be used to provide better access for emergency responders and to allow for traffic to be rerouted from a major incident. Design Criteria Important design considerations for installation of emergency crossovers on rural freeways include interchange spacing, median width, and stopping sight distance. Read the entire page →
From page 22... ... 20 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of emergency crossovers in reducing nonrecurrent congestion include the following: • Average reduction in response time for those situations in which the emergency crossover is used • Percentage of incidents (and incident type) for which the emergency crossover will be used to improve response time • Frequency of lane-blocking incidents for which queued traffic may be rerouted Cost Factors affecting the cost of installing an emergency crossover include the following: • Width and topography (cross slope) Read the entire page →
From page 23... ... 21 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION barrier borrows a lane from operation as a travel lane and adds it to the space within the work zone, potentially allowing work to be conducted more efficiently during offpeak hours without adversely affecting travelers during peak hours. Although the most common applications of MTBs are to alleviate recurrent congestion due to unbalanced flow during peak periods and to safeguard workers during long-term construction projects, the potential benefits to nonrecurrent congestion are considered here. Read the entire page →
From page 24... ... 22 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The use of MTBs in response to incidents is a function of the anticipated or observed severity of the incident-related congestion to the time required to move the barrier. In other words, if an incident can be cleared in less time than it would take to move the barrier, then it would not be useful to move it. Read the entire page →
From page 25... ... 23 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors that affect the cost of installing and operating an MTB system include the following: • Length of roadway where MTB will be used • Type of barrier-moving machine used • Number of barrier-moving machines needed • Frequency of use of the system • Movable barrier installation costs • Changes to pavement markings • Additional signing required • Ongoing operating costs o Machine operator wages (or additional wages, if an MTB system is already in place but currently used for recurrent congestion only) o Fuel, maintenance, and repairs for barrier-moving machine Gated Median Barriers Description and Objective A gated median barrier consists of adding a gated section within a continuous median barrier. Read the entire page →
From page 26... ... 24 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Design Criteria The Federal Highway Administration (FHWA) report Facilitating Incident Management Strategies on Freeways (Parham et al. Read the entire page →
From page 27... ... 25 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Factors that affect the cost of installing a gated median barrier include the following: • Type of gated median barrier (manually operated, automated) • Operational and ongoing maintenance costs • Cost of removing existing barrier • Installation of new signing to discourage unauthorized use of turnaround Movable Cable Median Barriers Description and Objective A movable cable median barrier involves construction of a specially designed wire cable barrier system that can be removed to allow median crossovers. Read the entire page →
From page 28... ... 26 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.4. Standard three-strand movable cable median barrier. Read the entire page →
From page 29... ... 27 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Length of proposed barrier section • Existing barrier type (modular concrete, semirigid, nonmovable cable) • Removal costs for the existing median barrier • Installation costs for the movable cable median barrier • Expected frequency of use for emergency access or queue dissipation Extra-Height Median Barriers Description and Objective An extra-height median barrier (EHMB) Read the entire page →
From page 30... ... 28 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.5. Extra-height median barrier in Atlanta, Georgia. Read the entire page →
From page 31... ... 29 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Number of Gawking Events EHMBs reduce rubbernecking by obscuring drivers' view of incidents on the other side of the barrier (the opposite direction of travel) Read the entire page →
From page 32... ... 30 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Factors that affect the cost of implementing EHMBs along a roadway section include the following: • Construction type (new roadway or retrofit of existing roadway) • EHMB material (reinforced concrete, planted shrubs) Read the entire page →
From page 33... ... 31 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Typical Applications Mountable/traversable medians are primarily used to provide emergency access, as described above, while still providing a visual separation between opposing directions of travel. Various traversable and nontraversable median types are found in every metro politan area around the nation. Read the entire page →
From page 34... ... 32 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Existing median type (raised curb, TWLTL, none) • Proposed median type (TWLTL, mountable curb, painted median) Read the entire page →
From page 35... ... 33 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Design Criteria The AASHTO Green Book (2011) states that desirably, a vehicle on the shoulder should clear the edge of the traveled way by at least 0.3 m (1 ft) Read the entire page →
From page 36... ... 34 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Reduces Incident Response Time An accessible shoulder can be used for emergency vehicle access. In this case, emergency responders use the shoulder to bypass traffic congestion on the roadway and reach the crash site earlier. Read the entire page →
From page 37... ... 35 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Temporary traffic control costs during construction • Removal or addition of striping • Addition or reinstallation of rumble strips Drivable Shoulders Description and Objective A drivable shoulder can be temporarily used by mainline traffic as a travel lane. Drivable shoulders can be used to restore lost capacity by routing traffic around a laneblocking incident, such as a work zone or a crash. Read the entire page →
From page 38... ... 36 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.10. Bus-only drivable shoulder on I-35 in Minneapolis, Minnesota. Read the entire page →
From page 39... ... 37 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION rightmost lane of the roadway may be treated using drivable shoulders by diverting traffic around the incident on the right shoulder. Factors Influencing Treatment Effectiveness The following factors may influence the effectiveness of drivable shoulders at reducing nonrecurrent congestion: • Number of days that the shoulder will be used as a lane during each work zone • Percentage of incidents (by magnitude) Read the entire page →
From page 40... ... 38 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION having a full shoulder for one direction of travel and little or no shoulder for the opposing direction of travel, a full shoulder is provided for one direction for some distance, and then the full shoulder is shifted to the opposite direction for some distance. Alternating shoulders may also be implemented as a permanent treatment when one shoulder has been converted to a travel lane to meet increased demand, or where paved shoulders on both sides of an undivided roadway are narrow. Read the entire page →
From page 41... ... 39 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The Green Book does not provide guidance on the recommended length of each intermittent shoulder. However, lengths of a quarter mile to a half mile have been reported by highway agencies. Read the entire page →
From page 42... ... 40 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION accommodate a vehicle trying to reenter the roadway. This is especially true for alternating shoulders in the median, where the reentering vehicle will be merging into the fastest lane of traffic. Read the entire page →
From page 43... ... 41 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Barrier and impact attenuator costs (moving existing, adding new, or replacing) • Costs associated with removing and adding pavement marking • Temporary traffic control (discussed in more detail below) Read the entire page →
From page 44... ... 42 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Traffic volumes on the opposite roadway from the incident are expected to be relatively high or expected to become high during the anticipated duration of the incident. Site Conditions • It is possible to deploy the incident screen while maintaining at least 3.9 ft (1.2 m) Read the entire page →
From page 45... ... 43 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION o A series of portable bases to support one or more panels, the stability of which may be augmented by means of sandbags. • The size and weight of the screen bases must be such as to ensure the stability of the free-standing incident screen system. Read the entire page →
From page 46... ... 44 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Factors that may affect the cost of portable incident screens include the following: • Level of wind resistance needed • Type of screen (e.g., fabric screen with metal tube support structure, chain link fence with opaque covering, or other) • Screen purchase cost • Total length of screen needed • Screen maintenance and storage costs • Cost for multipanel connections • Cost for stabilizing hardware (e.g., sandbags) Read the entire page →
From page 47... ... 45 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION lengths to allow a vehicle to safely and smoothly leave the traveled way and merge back onto it. Turnouts are often provided in rural locations when queues of traffic typically form behind slow-moving vehicles, causing impeded vehicles to make risky passing maneuvers that may result in crashes. Read the entire page →
From page 48... ... 46 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Topography of roadside (especially in mountainous conditions where considerable cut or fill may be required) • Percentage of roadway segment that will have shoulder (e.g., 100% would be a continuous shoulder) Read the entire page →
From page 49... ... 47 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors Influencing Treatment Effectiveness Bus turnouts are effective in maintaining uninterrupted traffic flow along the main thoroughfare. They also potentially improve safety by removing stopped vehicles from the roadway. Read the entire page →
From page 50... ... 48 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Typical Applications CISs are typically used on high-volume urban freeways, where crash investigations on the roadway shoulder have the greatest potential to create congestion. Design Criteria It is important that a CIS be clearly identified through signing and other markings. Read the entire page →
From page 51... ... 49 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Lane-Blocking Time of Incidents By providing a preestablished relocation spot, this treatment can reduce the time during which disabled, crash-involved, or police vehicles remain in the roadway or on the roadway shoulder. This reduction in lane-blocking time reduces the congestion associated with these incidents and increases the reliability of the roadway. Read the entire page →
From page 52... ... 50 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of a CIS at reducing nonrecurrent congestion: • Expected number of crashes per year (by severity) • Proportion of crash-involved vehicles that are physically movable (by type) Read the entire page →
From page 53... ... 51 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The primary objective of locked gate access, as illustrated in Figure 3.17, is to allow emergency and utility vehicles to access the local street network or a specific site from the freeway in remote locations where ordinary access is not provided via freeway ramp interchanges. In the reverse direction, these access points could also prove beneficial if an emergency services agency or utility company would have an unusually circuitous route to access the freeway. Read the entire page →
From page 54... ... 52 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Incident Duration A gated emergency access point between standard freeway interchanges reduces the time during which disabled, crash-involved, or police vehicles remain in the roadway or on the roadway shoulder by decreasing the response time of emergency personnel. By reducing the lane-blocking time of an incident, the nonrecurrent congestion associated with that incident is reduced. Read the entire page →
From page 55... ... 53 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Factors that may affect the cost of installing emergency access between interchanges include the following: • Type of control to be used (manually operated gate, automated gate, temporary barricades) • Pavement surface to be used (gravel, asphalt, concrete) Read the entire page →
From page 56... ... 54 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION avoid) the situation in which a ramp queue backs up onto the freeway mainline. Read the entire page →
From page 57... ... 55 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Required lane width for buses or other special vehicles, if added lane is to be an HOV lane or restricted to certain vehicles • Available right-of-way • Potential use of existing shoulder • Space for right-of-way maintenance to occur outside of lanes • Space for law enforcement outside of lanes • Ramp meters, if in use or if planned for future use • Auxiliary lanes on the arterial or freeway to accommodate additional lanes on the ramp Whenever construction is needed to widen a ramp, practitioners may find it beneficial to complete additional work, if needed, while the ramp is being widened. Such work may include fixing geometric deficiencies, repairing the roadway surface, and posting additional signs. Read the entire page →
From page 58... ... 56 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Frequency of off-ramp queues backing up onto the freeway mainline • Frequency of treatment use • Percentage capacity increase to off-ramp movements associated with ramp widening • Frequency of ramp-related crashes that could have been avoided if more maneuveri ng space were available Off-ramps with high demand-to-capacity (d/c) ratios will tend to benefit most from this treatment, particularly where the available storage space on the ramp is limited, because a high d/c ratio creates a situation in which queues may frequently back up onto the mainline. Read the entire page →
From page 59... ... 57 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Temporary ramp closures may be done in response to one of the following sources of nonrecurrent congestion: (1) major incidents, (2) Read the entire page →
From page 60... ... 58 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Barricades are often used for temporary closures because they are a low-cost option for infrequent closures. However, as setting up and removing barricades can be labor intensive, they are less desirable when closures are expected to be implemented more frequently. Read the entire page →
From page 61... ... 59 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Duration of the mainline event. Ramp closures will have a more prominent effect when a mainline work zone or other incident is expected to reduce the roadway capacity for a significant period of time. Read the entire page →
From page 62... ... 60 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.21. Read the entire page →
From page 63... ... 61 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Traffic control provided by law enforcement is a common treatment at interchanges near arenas, stadiums, and concert halls when high volumes of traffic are arriving or leaving at the same time. Although police presence is not directly design related, certain design elements may provide assistance to police officers or help make their directions more clear. Read the entire page →
From page 64... ... 62 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Frequency of off-ramp queues backing up onto the freeway mainline • Frequency of treatment use (applicable specifically to treatments that require ongoing activity, such as manual control by law enforcement) • Capacity increase expected from a particular ramp treatment (e.g., the addition of a turn lane will increase capacity significantly more than a small adjustment to signal timings) Read the entire page →
From page 65... ... 63 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Ramp turn restrictions limit access to an arterial in order to increase capacity at the ramp terminal. This capacity increase is intended to eliminate (or preemptively avoid) Read the entire page →
From page 66... ... 64 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Queue Spillback Events Ramp turn restrictions reduce the occurrence of off-ramp queues backing up onto the mainline of the freeway. Such events can have a significant effect on freeway reliability, because the queue effectively blocks a freeway lane until it dissipates. Read the entire page →
From page 67... ... 65 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION DETOURS Improvements to Detour Routes Description and Objective Detour routes are designated roadways set up to carry traffic along a secondary route to bypass a congested location on the primary route. Detour routes typically connect to the primary route at locations in close proximity to the point of congestion both upstream and downstream. Read the entire page →
From page 68... ... 66 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Providing additional surface area with new pavement to increase the number of usable lanes • Restriping or resigning routes to accommodate additional volumes • Minor widening at intersections • Retiming signalized intersections to give additional green time to directions of travel with increased volumes • Accommodating trucks and other large vehicles that may not have used the detour route previously (e.g., by increasing weight limits of bridges or increasing turning radii) • Improving the road surface, limiting access, improving sight distance, and other improvements that may allow a higher speed limit • Reconstructing isolated bottlenecks • Using high-quality pavement markings and stripes for unfamiliar drivers to follow • Using upgraded size and retroreflectivity of warning signs due to increased traffic load • Adding stop and yield signs or even traffic signals if existing intersection controls are inadequate for increased traffic loads • Adding temporary lighting to reassure drivers that they are following correct detour route • Adding an asphalt overlay before detour implementation to accommodate increased traffic load if detour is located on a poor pavement surface • Using a partial detour diversion such as keeping semitruck traffic (left lane) Read the entire page →
From page 69... ... 67 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION it is determined that they are likely to be used frequently (near high-crash locations) or potentially for long periods of time (near locations susceptible to severe damage, such as bridges near fault lines) Read the entire page →
From page 70... ... 68 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.22. Runaway truck ramp. Read the entire page →
From page 71... ... 69 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The AASHTO Green Book (2011) provides guidance on where runaway truck ramps should be located. Read the entire page →
From page 72... ... 70 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Sandpile ramps consist of loose, dry sand that quickly decelerates trucks entering the ramp. Weather can affect the rolling resistance provided by the sand in this type of runaway truck ramp. Read the entire page →
From page 73... ... 71 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION for a runaway truck ramp, the advantages of the impact attenuators should outweigh the disadvantages. How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Incidents The FHWA Desktop Reference for Crash Reduction Factors (Bahar et al. Read the entire page →
From page 74... ... 72 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Construction of a gravel arrester-bed ramp • Construction of a gravel gravity ramp • Construction of a sand arrester-bed ramp • Clearing and grubbing • Grading • Installation of impact attenuators • Installation of new signs CONSTRUCTION Reduced Construction Duration Description and Objective Reduced construction duration is a treatment category that encompasses innovative techniques that can be implemented to reduce the duration of a work zone or other construction project. Such techniques may include total road closures, night work, and the use of innovative construction materials. Read the entire page →
From page 75... ... 73 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Duration of Work Zone: Nonpeak Construction Reducing the duration of a work zone will decrease the lane-blocking time (and therefore the duration of the diminished capacity) associated with that work zone. Read the entire page →
From page 76... ... 74 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Duration of each work zone • Construction solutions in regards to timing, sequencing, and management of the project • Percentage decrease of roadway capacity for each hour of the day while the work zone is in effect Cost Factors that may affect the cost of implementing a reduced construction duration plan include the following: • Type of change to the construction plan (e.g., using more expensive materials that can be placed faster, providing bonuses to contractors for finishing early, working at night only) • Traffic management plan • Design time and fees associated with creating a new construction plan • Contractor bonus for each hour completed ahead of schedule • Fast-forming materials • Additional materials or equipment required Improved Work Site Access and Circulation Description and Objective In many construction or maintenance operations, vehicles carrying materials must access the work site from the traveled roadway. Read the entire page →
From page 77... ... 75 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Temporary traffic-control zone design should consider work vehicle access to the work site. Graham and Burch (2006) Read the entire page →
From page 78... ... 76 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.25. Boulders in the right-of-way (alternative to wildlife fencing) Read the entire page →
From page 79... ... 77 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 3.26. Long tunnels and long bridges physically separate animals from vehicular traffic. Read the entire page →
From page 80... ... 78 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Design Criteria Wildlife fencing is one the most widely used design treatments to prevent or reduce WVCs. One of the most undesirable effects of wildlife fencing is that it creates a barrier between natural habitats of different species and can potentially have harmful ecological consequences. Read the entire page →
From page 81... ... 79 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Each method will provide a different level of WVC reduction. Treatments that keep most wildlife off the roadway will result in a very large reduction in WVCs; treatments that attempt to make drivers aware of the potential for WVCs may have a smaller impact. Read the entire page →
From page 82... ... 80 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Winter weather can impair motorists' visibility, cause loss of vehicle control, reduce sight distance at curves and intersections, obscure signs, reduce effective roadway width, and render safety barriers ineffective. Snow fences can mitigate the negative impacts of winter weather by improving visibility and providing drier pavement surface conditions on the travel way. Read the entire page →
From page 83... ... 81 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • What benefits would be derived by solving the problem? Would it improve safety for the traveling public or maintenance crews? Read the entire page →
From page 84... ... 82 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Length of fence • Leasing agreements or land purchase for fences placed on private property Snow fences can significantly reduce the cost of snow removal. Blowing and drifting snow can add substantially to the cost of snow and ice removal for maintenance crews. Read the entire page →
From page 85... ... 83 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION after the incident. Types of treatments that reduce the amount of blowing sand on the roadway could be effective in improving emergency personnel response time. Read the entire page →
From page 86... ... 84 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Berms made from wood chips or soil • Solid fences • Stacks of hay bales • Walls of concrete blocks or other structural materials Wind breaks provide partial wind blockage, as they are less solid and continuous than wind barriers. Examples of wind breaks include the following: • Porous fences (such as standard orange construction fence) Read the entire page →
From page 87... ... 85 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION stop the blowing sand and to provide protected areas for possible development of new vegetative wind breaks. Read the entire page →
From page 88... ... 86 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION This treatment will be most effective on roadways that have a significant number of blowing sand–related crashes each year and long average lane-blocking durations. The vehicle demand volume is also an important consideration, as the delay associated with a lane-blocking event on a roadway with a high d/c ratio will be greater than that of a roadway with a lower demand volume. Read the entire page →
From page 89... ... 87 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION the chemicals are dispersed along a roadway segment through nozzles, which can be embedded into the pavement or placed along the edges of the roadway. Anti-icing systems are commonly applied on bridge decks, which tend to freeze more quickly than other parts of the roadway, and horizontal curves, where traction is critical. Read the entire page →
From page 90... ... 88 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Annual number of ice-related crashes • Number of hours per year when ice covers the roadway (if no anti-icing system were used) • Number of hours per year during which the melting capacity of the system is exceeded, making the system ineffective Several studies have shown reductions in the frequency of wintertime accidents from 25% to 100% due to the installation of anti-icing technologies. Read the entire page →

From page 19...

... 17 3 CATALOG OF NONRECURRENT CONGESTION DESIGN TREATMENTS This chapter catalogs design treatments that can be considered for use in addressing nonrecurrent congestion. A detailed summary of each of the design treatments is provided that includes the following information: • Description and objective • Typical applications • Design criteria • How treatment reduces nonrecurrent congestion • Factors influencing treatment effectiveness • Cost The design treatments are classified on the basis of similarities with respect to function or location on the roadway.