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Publication Number: FHWA-HRT-04-091
Date: August 2004
Signalized Intersections: Informational Guide
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CHAPTER 8 — SYSTEM-WIDE TREATMENTS
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
8.0 System-wide Treatments
Treatments in this chapter apply to roadway segments located within the influence of signalized intersections and intersections affected by the flow of traffic along a corridor. They primarily address safety deficiencies associated with rear-end collisions due to sudden accelerating/decelerating; turbulence involved with midblock turning movements from driveways or unsignalized intersections; and coordination deficiencies associated with the progression of traffic from one location to another. Four specific treatments are examined:
8.1 Median Treatments
The median of a divided roadway is used for left turns, pedestrian refuge, access to properties on the other side of the road, and separation of opposing directions of travel. These purposes can conflict, and each use should be considered when design changes are proposed.
Median design contributes to safe and efficient operation of intersections, especially left-turn movements. Specifically, width and type are key factors in median design. The median provides a location for vehicles to wait for a gap in opposing traffic through which to turn; it also separates opposing directions of travel. Inappropriate median design may contribute to operational or safety problems related to vehicles turning left from the major road and vehicles proceeding through or turning left from the minor road.
Operational or safety issues that provide evidence that median design changes may be appropriate include spillover of left-turn lanes into the through traffic stream, rear-end or side-swipe crashes involving left-turning vehicles, inappropriate use of the median, and pedestrian crashes. Medians may also form an integral part of an overall access management plan, as discussed later.
8.1.3 Key Design Features
Width, channelization, end type, and pedestrian treatments are key features of a median design. The elements combine to provide storage for left-turning vehicles, guide turning vehicles through the intersection, and help pedestrians cross the street.
Medians physically separate opposing directions of travel, and provide a safety benefit by helping reduce occurrence of head-on collisions. It is possible that a median can be so narrow or so wide that its safety benefit is canceled by operational or safety problems created by an inappropriate width, as shown in figures 57 and 58.
The appropriateness of the use of raised or flush medians depends on conditions at a given intersection. Raised (curbed) medians should provide guidance in the intersection area but should not present a significant obstruction to vehicles. The design should be balanced between the desire for it to be cost effective to construct and maintain and for it to provide safe channelization. Raised medians should be delineated (such as with reflectors) if lighting is not provided at the intersection, since they are sometimes difficult to see at night. AASHTO recommends that flush medians are appropriate for intersections with:(3)
Where left-turn lanes are provided in the median, curbed dividers should be used to separate left-turn and opposing through traffic on medians 4.8 m (16 ft) wide or less. These dividers should be 1.2 m (4 ft) wide. Medians 5.4 m (18 ft) wide or more should have a painted or physical divider that delineates the movements. It is also recommended that the left-turn lane be offset to provide improved visibility with opposing through traffic. This treatment is discussed in more detail in chapter 12.
Median End type
AASHTO provides the following guidance for median ends:(3, p. 701)
A semicircle is an appropriate shape for the end of a narrow median. An alternative design is a bullet nose, which is based on the turning radius of the design vehicle. This design better guides a left-turning driver through the intersection, because the shape of the bullet nose reflects the path of the inner rear wheel. The bullet nose, being elongated, better serves as a pedestrian refuge than does a semicircular median end.
Medians greater than 4.2 m (14 ft) wide with a control radius of 12 m (40 ft) (based on the design vehicle) should have the shape of flattened or squared bullets to provide channelization, though the length of the median opening will be controlled by the need to provide for cross traffic.
The median end controls the turning radius for left-turning vehicles. It can affect movement of vehicles using that leg of the intersection both to turn left from the approach and to depart from the intersection on that leg after turning left from the cross street. A median nose that does not significantly limit the turning radius will help turning vehicles proceed through the intersection at higher speeds. This could contribute to efficient vehicular operations but could also create additional safety issues for pedestrians.
Median pedestrian Treatments
Careful attention should be given to pedestrian treatments at signalized intersections with medians, as these intersections tend to be larger than most. Two key treatments are discussed here: the design of the pedestrian passage through the median, and the design of the pedestrian signalization.
Pedestrian treatments at medians can be accommodated in two basic ways: a cut-through median, where the pedestrian path is at the same grade as the adjacent roadway; and a ramped median, where the pedestrian path is raised to the grade of the top of curb. Figure 59 shows the basic features and dimensions for each treatment. Note that if the median is too narrow to accommodate a raised landing of minimum width, a ramped median design cannot be used. If the median is so narrow that a pedestrian refuge cannot be accommodated, then the crosswalk should be located outside the median. Per ADAAG, all curb ramps, including those at median crossings, must have detectable warnings. Further discussion of pedestrian treatments at medians can be found in FHWA's Designing Sidewalks and trails for Access: Part II.(34)
Pedestrian signal treatments also depend on the width of the median and are summarized in figure 60.
8.1.4 Safety Performance
Provision of medians at intersections provides safety benefits similar to medians between intersections. One report has shown that at urban and suburban intersections, multiple-vehicle crash frequency increases as median width increases for widths between 4.2 m (14 ft) and 24 m (80 ft), unlike in rural areas where multiple-vehicle crash rates tend to be lower for wider medians.(79) The report also provided a summary of a study that found no statistically significant effect of median width on traffic delays and conflicts on medians between 9 m (30 ft) and 18 m (60 ft) wide.(80)
One study found decreasing crash rates with increasing median widths.(81) A Michigan State University study found that Michigan's boulevard roadways experience a crash rate half that of roadways with continuous center left-turn lanes.(82) A median width of 9.15 m (30 ft) to 18.30 m (60 ft) was found to be the most effective in providing a safe method for turning left.
8.1.5 Operational Performance
Simulation of signalized directional crossovers showed they operate better than other designs (specifically, an undivided cross section with a continuous center left-turn lane and a boulevard with bidirectional crossovers). The undivided cross section has larger delays for left-turning vehicles than do boulevard roadways, even for low turn volumes. The width of the median affects the storage capacity of the crossover, of course, so a crossover in a narrow median may not function as well as a left-turn lane. The signalized crossovers functioned more efficiently (i.e., with less time to make a left-turn) than did stop-controlled crossovers.(83)
8.1.6 Multimodal Impacts
As noted previously, the width of the median (and the roadway in general) has a direct relationship with the amount of time needed for pedestrians and bicycles to cross the roadway. Large intersections that have no median or a median too narrow to provide a refuge force pedestrians to cross the entire street in one stage. Therefore, the provision of a median with at least enough width to accommodate a pedestrian can provide pedestrians with the option of crossing in one stage or two. This can be a significant benefit to elderly and disabled pedestrians who cross at speeds less than the typical 1.2 m/s (4 ft/s) or 1.1 m/s (3.5 ft/s) used to time pedestrian clearance intervals.
If the median is so wide that pedestrian crossings are operated in two stages, the sequence of the stages may increase crossing time significantly. For example, if the vehicle phases running parallel to the pedestrian crossing in question are split-phased and the sequence of the vehicle phases is in the same direction as the pedestrian, crossing time is similar to that of a single-stage crossing. On the other hand, the reverse direction will result in additional delay to the pedestrian in the median area as the signal cycles through all conflicting phases.
8.1.7 Physical Impacts
Improvements made in the median should not have an effect on the footprint of an intersection unless a roadway is widened to provide the median to use for left-turn lanes, pedestrian refuges, and so on. Use of a curbed median or separator between left and opposing through traffic would add a vertical component to the intersection that should accommodate the necessary curb cuts.
8.1.8 Socioeconomic Impacts
The primary socioeconomic impact of medians at signalized intersections relates more to their effect on overall access within the corridor, which is discussed in section 8.2. However, landscaping can play an important aesthetic role at the intersection itself. The appropriate use of landscaping can visually enhance a road and its surroundings. Landscaping may act as a buffer between pedestrians and motorists, and reduce the visual width of a roadway, serving to reduce traffic speeds and providing a more pleasant environment.
Landscaping must be carefully considered at signalized intersections, otherwise it will prevent motorists from making left and right turns safely because of inadequate sight distances. Care should be taken to ensure that traffic signs, pedestrian crossings, nearby railroad crossings, and school zones are not obstructed. Median planting of trees or shrubs greater than 0.6 m (2 ft) in height should be well away from the intersection (more than 15 m (50 ft)). No plantings having foliage between 0.6 m (2 ft) and 2.4 m (8 ft) in height should be present within sight distance triangles.
Low shrubs or plants not exceeding a height of 0.6 m (2 ft) are appropriate on the approaches to a signalized intersection, either on the median, or along the edge of the roadway. These should not be allowed to overhang the curb onto the pavement nor interfere with the movement of pedestrians. All planting should have an adequate watering and drainage system, or should be drought resistant. FHWA's report Vegetation Control for Safety provides additional guidelines and insight.(84)
8.1.9 Enforcement, Education, and maintenance
Medians introduce little in the way of unique enforcement or education issues for motor vehicles. Pedestrians may need assistance through the use of signs or other methods to make them aware of one-stage versus two-stage crossings, particularly in communities that have both types of crossings at their signalized intersections.
Table 38. Summary of issues for providing median treatments.
8.2 Access Management
Practical experience and recent research indicate that controlling access on a roadway can have a positive impact on both traffic operation and safety. Access management is a key issue in planning and designing roadways so they perform according to their functional classification.
The topic of access management is growing and exceeds the space that this guide can provide. More information on access management can be found in a number of references, including AASHTO's a policy on Geometric Design of Highways and Streets;(3) NCHRP 420: Impacts of Access Management techniques;(85) ITE s Transportation and Land Development;(86) and TRB's Access Management Manual.(87) Several States, including Colorado and Florida, also have extensive guidance on access management. This section focuses on the operational and safety effects of unsignalized intersections (both public streets and private driveways) located within the vicinity of signalized intersections.
Access management plays an important role in the operation and safety of arterial streets where both mobility of through traffic and access to adjacent properties are needed. Studies have repeatedly shown that improvements in access management improve safety and capacity, and also that roadways with poor access management have safety and operations records worse than those with better control of access. Treatments to improve access management near intersections (within 75 m (250 ft) upstream or downstream) include changes in geometry or signing to close or combine driveways, provide turn lanes, or prohibit turn movements.
AASHTO presents a number of principles that define access management techniques:(3)
Access management works best when combined with land use and zoning policies. Parking lot placement behind urban and suburban shopping and community attractions can minimize the need to mitigate traffic movements.
Intersection problems that indicate an improvement in access management may be desirable include delay to through vehicles caused by vehicles turning left or right into driveways, and rear-end or angle crashes involving vehicles entering or leaving driveways.
8.2.3 Design Features
To understand the effects of a signalized intersection on access management upstream and downstream of the intersection, the functional area of the signalized intersection, shown in figure 62, needs to be determined. The functional area is larger than the physical area of the intersection because it includes several items, as shown in figure 63:(87)
When two signalized intersections are in proximity to each other, their overlapping functional areas may result in varying levels of access that might be considered between the two intersections, as shown in figure 64. As the figure demonstrates, the functional areas of nearby signalized intersections affects the location and extent of feasible access. Ideally, driveways with full access should be located in the area clear of the functional areas of both signalized intersections. However, signalized intersections are often located close enough to each other that the upstream functional area of one intersection partially or completely overlaps with the upstream functional area of the other. In these cases, there is no clear area between the two intersections where a driveway can operate without infringing upon the functional area of one of the signalized intersections. As such, it is important to apply sound engineering judgment to determine where and if driveway access should be allowed. Some important considerations in the evaluation would include the volume of traffic using the driveway, the type of turning maneuvers that will be most prominent, the type of median present, potential conflicts with and proximity to other driveways, and the volume of traffic on the major street.
Access points that are clear of only one of the two signalized intersections would likely perform best from a safety perspective if restricted to right-in, right-out operation. However, in urban areas, this may not always be practical or may create other problems at downstream intersections, so again it is important to apply sound engineering judgment. In some cases, the two signalized intersections may be so close together that any access would encroach within the functional area of the intersection. These situations are likely to be candidates for either partial or full access restriction. It is important to note that driveways should not be simply eliminated based on simple guidelines but rather should be evaluated on a case-by-case basis with consideration of the broader system effects. When driveways are closed without any regard to the system effects, there is a high potential that the problem will be transferred to another location. Finally, as a general guideline, the functional area of an intersection is more critical along corridors with high speeds (70 km/h (45 mph) or greater) and whose primary purpose is mobility.
Improvements to the current access to properties adjacent to an intersection area can be implemented by:
As discussed previously, where access is restricted, the redirection of driveway traffic needs to be considered. Two of the more typical options are:
Note that the conversion of an existing full-access point to right-in/right-out operation has both advantages and disadvantages. The advantages of right-in/right-out operation include:
8.2.4 Safety Performance
In general, an increase in the number of access points along a roadway correlates with higher crash rates. Specific relationships vary based on specific roadway geometry (lane width, presence or absence of turn lanes, etc.) and traffic characteristics.
Table 39 presents a summary of the relative crash rates for a range of unsignalized intersection access spacing. As can be seen, doubling access frequency from 6 to 12 access points per km (10 to 20 access points per mi) increases crash rates by about 40 percent. An increase from 6 to 37 access points per km (10 to 60 access points per mi) would be expected to increase crash rates by approximately 200 percent. Generally, each additional access point per mile along a four-lane roadway increases the crash rate by about 4 percent (see also references 89 and 90).
Table 39. Relative crash rates for unsignalized intersection access spacing.*
*Source: Reference 87, as adapted from 85.
**Total access connections on both sides of the roadway.
*** Average spacing between access connections on the same side of the roadway; one-half of the connections on each side of the roadway.
8.2.5 Operational Performance
The reduction of access along an arterial street has the potential to improve traffic operations. For example, urban arterials with a high degree of access control function 30 to 50 percent better than the same facility with no control.(91) Improved access management also has been shown to improve LOS.(92)
Access points close to a signalized intersection can reduce the saturation flow rate of the signalized intersection. Research has determined that the amount of reduction depends on the corner clearance of the driveway, the proportions of curb-lane volume that enter and exit the driveway, and the design of the driveway itself.(93)
However, as indicated earlier, it is important to evaluate the impact of access control on the upstream and downstream intersections, which may experience a significant increase in U-turns or other types of turning movements. For example, if there is adequate capacity to accommodate the turning movements at midblock access driveways and no safety problems have been identified, eliminating the left-turn movements and converting them to U-turns at signalized intersections would likely degrade the operational performance of the arterial because less green time will be available for through traffic.
8.2.6 Multimodal Impacts
Access treatments that reduce the number of driveways or restrict turning movements at driveways also reduce the number of potential conflicts for pedestrians and bicycles near a signalized intersection. In addition, a median treatment used as part of an overall access management strategy may also provide the opportunity for a midblock signalized or unsignalized pedestrian crossing. It would be important to evaluate whether the access treatments being considered would result in a significant increase in operating speed on the facility, as increases in speed have a negative impact on both pedestrians and bicyclists that should be considered in the evaluation.
8.2.7 Physical Impacts
Addition of turn lanes for property access will increase the footprint of the intersection area. Turn restrictions should not have any effect on the physical size of an area, but may add a vertical element to the intersection (if, for example, a raised curb or flexible delineators are used to prohibit left turns). These should not present difficulties for pedestrians with mobility impairments, nor obstruct sight distance.
8.2.8 Socioeconomic Impacts
A review of the literature indicates inconsistency in the socioeconomic effects of access management. Surveys conducted in Florida reported a relatively low rate of acceptance of access management: most drivers felt that the inconvenience of indirect movements offset the benefits to traffic flow and safety. Businesses also were unsupportive: 26 percent reported a loss in profits, and 10 to 12 percent reported a large loss.(94) Conversely, experience in iowa indicates rapid growth in retail sales after access management projects were completed. An opinion survey conducted among affected motorists indicated that a strong majority supported all projects but one.(92)
The reactions of drivers, property owners, pedestrians, and others concerned with access to properties adjacent to intersections would be expected to vary widely. Access management strategies should be considered only in the context of a roadway corridor with the approval and backing of those affected.
Relocation or closing of driveways should be part of a comprehensive corridor access-management plan. The optimal situation is to avoid driveway conflicts before they develop. This requires coordination with local land use planners and zoning boards in establishing safe development policies and procedures. Avoidance of high-volume driveways near congested or otherwise critical intersections is desirable.
Highway agencies also need to have an understanding of the safety consequences of driveway requests. The power of a highway agency to modify access provisions is derived from legislation that varies in its provision from State to State. Highway agencies generally do not have the power to deny access to any particular parcel of land, but many do have the power to require, with adequate justification, relocation of access points. Where highway agency powers are not adequate to deal with driveways close to intersections, further legislation may be needed.
8.2.9 Enforcement, Education, and maintenance
Periodic enforcement may be needed to ensure that drivers obey restrictions at driveways where such restrictions cannot be physically implemented with raised channelization, such as signed prohibitions.
Table 40. Summary of issues for providing access management.
drivers may have difficulty making permissive turning maneuvers at signalized intersections (e.g., permissive left turns, right turn on red after stop) because of lack of gaps in through traffic. This can contribute to both operational and safety problems. Left-turning vehicles waiting to turn can block through traffic, even if a left-turn lane is provided. This can lead to rear-end crashes between turning and through vehicles. Collisions may also occur when left-turning drivers become impatient and accept a gap that is smaller than needed to complete a safe maneuver. Such collisions could be minimized if longer gaps were made available. (This could also be accomplished through turn prohibitions and changes in signal phasing, although this particular treatment does not address these potential countermeasures.)
One method of providing longer gaps is to coordinate adjacent traffic signals to promote platooning of vehicles. Signals within 0.8 km (0.5 mi) of each other on a major route, or in a network of major routes, should be coordinated; signals spaced farther than 0.8 km (0.5 mi) may be candidates for coordination if platooning can be maintained. Signal progression can help improve driver expectancy of changes in right-of-way assignment due to signal changes. Increased platooning of vehicles can create more defined gaps of increased length for permissive vehicle movements at intersections and can result in improved intersection operation. Increased platooning of vehicles may also result in a decrease in rear-end crashes. Effective coordination of signals should reduce the required number of stops for the higher priority movements (presumably the major street through movement).
Signal coordination may be applicable for intersections where:
8.3.3 Safety Performance
Apart from its operational benefits, signal coordination is known to reduce vehicle conflicts along corridors where traffic signals are coordinated. Largely, it reduces the number of rear-end conflicts, as vehicles tend to move more in unison from intersection to intersection.
Studies have proven the effectiveness of signal coordination in improving safety. The ITE Traffic Safety Toolbox: a primer on Traffic Safety cites two studies of coordinated signals with intersection crash frequencies that dropped by 25 and 38 percent.(95) One study showed a decrease in crash rates for midblock sections as well. A study on the effectiveness of traffic signal coordination in arizona concluded that there is a small but significant decrease in crash rates on intersection approaches after signal coordination.(96) Crashes along the study corridor decreased 6.7 percent. Another study of the safety benefits of signal coordination carried out in phoenix compared coordinated signalized intersections to uncoordinated signalized intersections citywide. The coordinated intersections were found to have 3 to 18 percent fewer total collisions, and 14 to 43 percent fewer rear-end collisions.(97)
Table 41. Selected findings of safety benefits associated with signal coordination or progression.
8.3.4 Operational Performance
The potential benefits of coordination are directly related to the traffic characteristics and spacing of intersections. Coordinated operation works best when traffic arrives in dense platoons. These platoons occur more frequently when the intensity of traffic volume between intersections increases and distance between intersections decreases, to a practical limit. Selection of the system cycle length defines the relationship that allows coordinated operations between the intersections, while the offset represents the difference in start times for the through green at adjacent intersections.
A key to success in signal coordination is the appropriate spacing of the signals. Signals within a half-mile (or sometimes even more if platooning can be maintained) of each other should be coordinated. As with all signals, coordinated signals too close together can present problems when drivers focus on a downstream signal and do not notice the closer signal they are approaching, or the proceed through a green signal and are not able to stop for a queue at a signal immediately downstream. Dispersion of platoons can occur if signals are spaced too far apart, resulting in inefficient use of signal coordination and loss of any operational benefit. Operations on cross streets may be negatively impacted. The Colorado Access Demonstration project concluded that 0.8-km (0.5-mi) spacing could reduce vehicle hours of delay by 60 percent and vehicle-hours of travel by over 50 percent compared with signals at one-quarter mile intervals with full median openings between signals (reference 87, adapted from reference 99).
Grouping the signals to be coordinated is a very important aspect of design of a progressive system. Factors that should be considered include geographic barriers, volume-to-capacity ratios, and characteristics of traffic flow (random versus platoon arrivals). When systems operating on different cycle lengths are adjacent to or intersect each other, changes to provide a uniform cycle length appropriate for both systems should be considered, so that the systems can be unified, at least for certain portions of the day. Half-cycles or double cycles should also be considered for some locations if that facilitates coordination.
8.3.5 Multimodal Impacts
Signal coordination, by providing for a more orderly flow of traffic, may aid pedestrians in anticipating vehicle movements, lessening the likelihood of a pedestrian-vehicle conflict. In addition, in some cases it may be possible to provide progression for pedestrians in one or both directions along with vehicle progression.
8.3.6 Physical Impacts
8.3.7 Socioeconomic Impacts
8.3.8 Enforcement, Education, and maintenance
Signals working in coordination should reduce excessive speed, as motorists realize that they cannot "beat" the next traffic signal. Incidents of aggressive driving should be reduced as well.
Table 42. Summary of issues for providing signal coordination.
One difficulty in understanding preemption and priority is the lack of standard terms in current practice and the multiple approaches that have been employed to date. The recent development in the signal Control and Prioritization (SCP) standard has provided some guidance, but some detail will be provided here.
Preemption is primarily related to the transfer of the normal control (operation) of traffic signals to a special signal control mode for the purpose of servicing railroad crossings, emergency vehicle passage, mass transit vehicle passage, and other special tasks, the control of which requires terminating normal traffic control to provide the serve the special task.
Priority is defined by the preferential treatment of one vehicle class (such as a transit vehicle, emergency service vehicle, or a commercial fleet vehicle) over another vehicle class at a signalized intersection without causing the traffic signal controllers to drop from coordinated operations. Priority may be accomplished by a number of methods, including changing the beginning and end times of greens on identified phases, changing the phase sequence, or including special phases, all without interrupting the general timing relationship between specific green indications at adjacent intersections.
8.4.2 Emergency Vehicle Preemption
A specific vehicle often targeted for signal preemption is the emergency vehicle. Signal preemption allows emergency vehicles to disrupt a normal signal cycle to proceed through the intersection more quickly and under safer conditions. The preemption systems can extend the green on an emergency vehicle s approach or replace the phases and timing for the whole cycle. The MUTCD discusses signal preemption, standards for the phases during preemption, and priorities for different vehicle types that might have preemption capabilities.(1)
Several types of emergency vehicle detection technologies are available, and include the use of light, sound, pavement loops, radio transmission, and push buttons to detect vehicles approaching an intersection:
Many of these systems have applications in transit-vehicle priority as well as signal preemption for emergency vehicles. Some jurisdictions use signs that alert drivers that a police pursuit is in progress.
Preemption/priority is considered where:
8.4.4 Safety Performance
No research is known on the safety implications of emergency vehicle preemption, although it is expected that the number of conflicting movements associated with an emergency vehicle s having to run a red light would be reduced.
Installation of signal preemption systems for emergency vehicles has been shown to decrease response times. A review of signal preemption system deployments in the United States shows decreases in response times between 14 and 50 percent for systems in several cities. In addition, the study reports a 70 percent decrease in crashes with emergency vehicles in St. Paul, MN, after the system was deployed.(100)
Signal preemption has also been considered for intersections at the base of a steep and/or long grade. These grades can create a potentially dangerous situation for large trucks if they lose control and enter the intersection at a high speed. Preemption could be used to reduce the likelihood of conflicts between runaway trucks and other vehicles.
8.4.5 Operational Performance
Preemption of signals by emergency vehicles will temporarily disrupt traffic flow. Congestion may occur, or worsen, before traffic returns to normal operation. Data gathered on signal preemption systems in the Washington, DC, metropolitan area suggested that once a signal was preempted, the coordinated systems took anywhere between half a minute to 7 minutes to recover to base time coordination. During these peak periods in more congested areas, vehicles experienced significant delays. Agency traffic personnel indicated that signal preemption seems to have more impacts on peak period traffic in areas where the peak periods extend over longer time periods than it does where peak periods are relatively short.(100)
8.4.6 Multimodal Impacts
8.4.7 Physical Impacts
The key to success is ensuring that the preemption system works when needed by providing clear sight lines between emergency vehicles and detectors. Also, it is important to ensure that vehicles from a variety of jurisdictions will be able to participate in the signal preemption program.
Light-based detectors need a clear line of sight to the emitter on the vehicles; this line could become blocked by roadway geometry, vehicles, foliage, or precipitation. Also, systems from different vendors may not interact well together. Other alarms, such as from nearby buildings, may be detected by a sound-based system.
8.4.8 Socioeconomic Impacts
8.4.9 Enforcement, Education, and maintenance
Preemption directly benefits emergency vehicles, although most police agencies do not use signal preemption. Preempted signals that stop vehicles for too long may encourage disrespect for the red signal, although this has not been reported.
Table 43. Summary of issues for providing signal preemption and/or priority.