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Traffic Management

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  1. Traffic Management Hossein Naraghi CE 590 Special Topics Safety January 2003 Time spent:13 hrs

  2. Road network and functional hierarchy • Traffic management is a process of adjusting or adapting the existing road network to improve traffic operations without major construction • Traffic management objectives may include • Traffic efficiency • Road capacity • Improved environment amenity • Reduced noise

  3. Traffic management objectives (continued) • Enhanced access • Better access for particular group of road users • Pedestrians • Bicyclists • Freight vehicles • Road safety These objectives may potentially be in conflict of each other, so priorities may have to be determined.

  4. Road functions and functional hierarchy • The use of traffic management to pursue safety objectives must take place in the context of a clear view of the functions of a road network, which is referred to as functional hierarchy. • Road’s function is the prime determinant of the management of any given road within the overall network.

  5. Road functions and functional hierarchy (continued) • There are essentially two needs from a road function point of view • The traffic movement function • The role of roads in providing a means to transfer people and goods from one place to another; these roads constitute the arterial road network • The access function • The role of these roads is providing access to abutting properties and land uses; these roads constitute the local road network Ideally each road would perform one of these functions, but in practice there are many roads that perform both functions

  6. Road functions and functional hierarchy (continued) • The only roads that do not have an access function are those which are access controlled • The only access is via ramps at interchanges • Freeways • Access from abutting property is oriented away from the road • Sometimes done with distributor roads in residential neighborhoods • Access via frontage roads paralleling an arterial roads

  7. Road functions and functional hierarchy (continued) • The roads which carries both functions of access and mobility, create a major challenge for traffic management • Tend to have very poor crash record as a result of their mixed and inherently conflicting functions • People living and working along them seek to use them for • Access purposes • Exiting and entering properties • Parking in the street • Have significant pedestrians and bicycle activities in residential areas

  8. Road hierarchy as a network planning tool • The basis for any traffic management plan is usually the development of road hierarchy and agreement to it by various stakeholders. • Main objectives • Prevent residential areas being used by through traffics • Influence driver behavior to follow planned routes at moderates speeds, with proper attention being given to pedestrians and bicyclists • Use physical devices in support of legal regulations in order to overcome the lack of enforcement for such measures as speed limits, one way streets and turning prohibitions.

  9. Road hierarchy as a network planning tool (continued) • Local roads, which have solely an access function, will have objectives related to local amenity and safety • If road configuration does not allow these objectives to be met, then there is a need for some form of treatment • Similarly roads with mixed functions will need some kind of treatment in order to satisfactory achieve both safety and mobility • There is also an urban space which is motor vehicle-free • Mobility is solely provided by walking and bicycling

  10. Road hierarchy as a network planning tool (continued) • Gunnarsson defines three classes of road • F (exclusive foot space) • C (local street) • T (exclusive transport space) This provides a conceptual framework for traffic calming F C T F/C T/C Gunnarson describes local street (zone C) and two transition zones (F/C and T/C) as “traffic calming space”

  11. Lessons for new network • Wallwork (1993) said: • “Traffic calming is a negative reaction to a problem caused by bad planning, zoning and/or street design. We need to be proactive in our approach and learn from the past and others” • This is mainly involve • Attention to network layout • Ensure that problems are not built in • Attention to detail in civic design • To ensure that an appropriate balance of built and open spaces and the connection between them are provided

  12. Lessons for new network (continued) • Research for the safety aspects of road network design summarized the key network planning principles for new residential areas as follows • Strict differentiation of streets according to their traffic function leads to safer residential areas • Distribution of traffic into residential area with multiple access from a ring road is safer than central distribution

  13. Lessons for new network (continued) • Full segregation of vehicle, pedestrians and bicycle movements is accompanied by very low accident rates • Cul-de-sac streets are safer than loop streets, which in turns are safer than ordinary through streets • Crash rate is minimized where frontage access is prohibited

  14. Traffic management or traffic calming • There have been many different interpretations of traffic calming in different countries • A useful resolution of this conflict of interpretation has been proposed by Brindle • Traffic calming is used encompasses two axes • The scope of the measure • Local, intermediate or citywide • The type of measure • Use of physical devices and regulations • Social/cultural change

  15. Area treatments • Area treatments divided into two distinct types • Applicable to low density residential development • Aim to control speeds which are in excess of a statutory speed limit of around 30-35 mph • Used in higher density cities • Aim is to reduce speed to around 20 mph

  16. Area treatments (continued) Safety effectiveness • Traffic management schemes are usually successful in reducing • Vehicle speeds • Sometimes traffic volumes • But whether they increase objective safety; as measured by accident, is open to question • An in depth analysis of accident pattern within case study area in Sydney concluded that • Many of traffic management devices used extensively in local area fail to resolve the dominant types of accidents occurring on local streets

  17. Area treatments (continued) • The dominant types of accident were • Cross traffic • Right-through • Rare end • Hit parked vehicles Local area traffic management schemes may not be as effective in reducing accidents as many practitioners believe However, there were several studies which have revealed the positive effects of traffic management schemes

  18. Area treatments (continued) Environmentally adapted through roads • This is one which retains its status as an arterial road • Traffic efficiency role is curtailed, so other environmental or amenity goals may also be realized • There are two typical application • Bypass towns • Adaptation of roads still fulfilling their arterial function

  19. Area treatments (continued) • In either case, treatments typically involve some or all of the following • A form of gateway treatment using signs and town entry features to • Emphasize the changed status of the road • Encourage slower driving • Provide a sense of identity • Speed restricting devices • Road narrowing • Road humps • Enhanced pedestrians and bicyclists facilities

  20. Area treatments (continued) • Improved parking especially for bypass towns • The town can rejuvenate as a business center when through traffic is removed • Change traffic furniture like lighting and seating • Enhanced signing • A roundabout at the start and end of the town • To ensure traffic slows • Narrower lanes • Change in road surface including color and texture • Additional traffic control measures • Traffic signal • Pedestrian crossing

  21. Area treatments (continued) Problems and difficulties • There is a negative response from affected parties which needs to be considered • Brindle (1992) reported a citizen group opposition to traffic calming (especially speed humps) in the following terms • They discriminating against law-abiding road users • They are dangerous to bicyclists and motor cyclists, especially when wet

  22. Problems and difficulties (continued) • They cause unnecessary wear and tear on motor vehicles • They disadvantage some local businesses • Their lighting is a source of annoyance to some residents • They hinder emergency vehicles • They waste taxpayer’s money • They make some motorists more aggressive and impatient • They detract from the environment • lights, signs, noise

  23. Problems and difficulties (continued) • They devalue property • They are inappropriate for heavy vehicles • They increase road maintenance costs • They cost people time Some of these points are not without validity considerations such as these mean that the development of traffic calming schemes requires careful design and sensitive implementation

  24. Speed management • Speed may be managed in two ways • Traffic calming • Using physical devices aimed to restrict the speed of vehicles • Imposition of speed limits Speeds and safety • There is clear evidence of the effect of speed on accident rates • As travel speeds drop, the impact speeds drop, and collision may be avoided

  25. Speed management (continued) • Organization for Economic Cooperation and Development has quantified the effect of speed on accidents and accident severity, based on Swedish data • The percentage drop in accident rates outside built up area is n times the percentage drop in mean speed • n = 4 for fatal accidents • n = 3 for injury accidents • n = 2 for all accidents • In urban areas, there are two distinct speed related accident problems • Injury to drivers and passengers of vehicles where speed is in excess of posted speed limits

  26. Speed management (continued) • Injury to vulnerable road users • Pedestrians and bicyclists • In many cases the driver is within the speed limit • McLean, et al, 1994 based on a detailed study of 146 fatal pedestrian crashes found that 45 percent of these would probably survived if vehicle struck them with 10 km/h slower speed Speed Limits • Speed limits affect travel speed and therefore should affect accidents specially in urban areas and roads with limited design standards • There is less clear evidence of affect of speed limits on crashes in rural areas • Different studies found different results on affect of increasing speed limit in rural areas

  27. Speed limits and travel speeds • Speed limits affect safety only if they affect actual travel speed • The influence of speed limit relies • Reasonableness of speed limit as perceived by driver • On enforcement Reasonableness of speed limits • ‘In order to bring about a reduction in mean speed and speed dispersion, a speed limit should be set at 85 percentile of existing speeds, or at a lower level ( but not too far below)’

  28. Reasonableness of speed limits (continued) • Road environment factors affecting the perception of driver about the reasonableness of speed limit • Alignment • Urban or rural environment • Road category • Lane width • Roadside development • Traffic density • Sight distance • Parked vehicles • Pedestrians • Day and night vision

  29. Enforcement • Enforcement would result in a reduction in mean speed and in the spread of speed • This will lead to reduction in crash number and severity • Automated speed enforcement have been effective in reducing speed • Not only on the site where they have been set up • But also in leading to a change in attitudes towards speeding • The wide spread use of speed cameras has already produced a change in driver attitude in Australia where portable speed cameras have been in use for several years

  30. Differential speed limits for heavy vehicles • ‘There is evidence that accident rates are related to the dispersion or variance of speeds of vehicles in the traffic stream’ • Many different studies reported the chance of being involved in an accident follows a U-shaped distribution (Figure 13.4 page 335) • The minimum occurring when the vehicle is traveling at about the average speed of traffic, or slightly above • As vehicle speeds move significantly above or below the average speed, the probability of being involved in a crash increases dramatically

  31. Differential speed limits for heavy vehicles (continued) • There is an argument that there should not be a speed limit differential between heavy vehicles and other road vehicles • Evidence to support this produced in US following the introduction of nationwide 55 mph car speed limits in 1974, the observed speed differential between cars and trucks was reduced • Radwan and Sinha examined the effect of this on truck accidents and found • There had been reduction in heavy truck crash rates in all severity types (fatality, injury and PDO) on freeways • On four-lane and two-lane rural highways, the reduction occurred only for injury crashes • The reductions were attributed to an absolute reduction in speeds and decreased speed dispersion

  32. Setting speed limits • There are four types of speed limits • General limits • Impose by statute and are applicable to all roads in an area unless signed otherwise • A general urban or rural limit • Speed zones • Speed limits applied to a specific road depends on the road’s design characteristics and its traffic and land use characteristics • Varying by time of day

  33. Setting speed limits (continued) • Vehicle limits • Apply to specific classes of vehicle • Trucks and buses • Driver limits • Apply to specific classes of driver • Learners Formal management of vehicle speeds using any of these legally enforceable speed limits involves • Establishing a balance between safety, mobility, and amenity for users

  34. Setting speed limits (continued) • Meeting driver expectation • Making speed limits more or less self enforcing • Achieving consistency across the jurisdiction • Ability to deter offenders by appropriate levels of enforcement • Developing a culture of compliance • Minimizing the cost of signing and enforcement • A systematic approach to setting speed limits in a speed zoning context is the expert system called VLIMITS developed by the Australian Road Research Board which considers the factors shown in Table 13.1 page 339

  35. Setting speed limits (continued) • VLIMITS is used as a basis for a major statewide speed limit review in Victoria • The guidelines to use various speed limits are as follow • Shared zone (6 mph) • Vehicles and pedestrians sharing the same road space • Service vehicles in shopping malls • Local traffic area zone (24 mph) • Traffic management works have been undertaken to physically limit the speed • Local street speed limit (30 mph) • Local access or collector roads with abutting development

  36. Setting speed limits (continued) • General urban limit (36 mph) • Applies to all urban roads that do not meet the criteria for higher or lower speed limit • Undivided or divided arterial roads with substantial abutting development • Collector roads with higher standard • 25 ft between curbs and carrying above 5000 vpd • Urban (42 mph) • Divided roads with direct access to through roadway • Undivided roads with low traffic volumes, little or no abutting development

  37. Setting speed limits (continued) • Urban (48 mph) • Divided roads with substantial abutting development with little or no direct access • Undivided roads with limited amount of abutting development • General rural limit (60 mph) • Little or no abutting development and widely spaced intersection • Rural freeway (66 mph) • Applies to high standard rural freeways • meet the current design standards • Have a 30 ft roadside clear zone • Accident rate of less than one fatal accident per 1.2 miles per year • Widely spaced interchanges (greater than 2 miles)

  38. Devices and techniques for managing speed and volume • Type of devices and techniques available for keeping speed low falls into six categories • Regulatory devices • Speed limits • Stop and yield signs • No-turn signs • One-way operation • Network modifications • Street closure at intersection • Link closure

  39. Devices and techniques for managing speed and volume (continued) • Partial street closure • Diagonal closure of intersection • Placement of a barrier diagonally across a cross-intersection, to create two right-angle bends instead of an intersection • Closure of median opening • Pedestrian refuge and/or narrow median • Devices used at intersections • Roundabouts • Resolves priority issues in local streets • Vehicle is required to divert from a straight line • Reduce vehicle speed

  40. Devices and techniques for managing speed and volume (continued) • Klyne (1988) established an empirical relationship between speed and path radius V=6√R/S V = 95 percentile speed (km/h) of through vehicles R = radius of centerline of vehicle path (meters) S = sight distance factor (S=1.0 for good sight distance, up to 1.53 for poor sight distance This formula suggests that to keep 95 percentile speeds through an intersection to 30 km/h, the roundabout geometry should be such that the path radius developed can not be greater than 80 feet

  41. Devices and techniques for managing speed and volume (continued) • Channelization • To reallocate priority at an intersection • At t-intersection to give priority to vehicles traveling around a curve • To deter speeding by motorists on the top of the t-intersection by requiring them to slow down to negotiate the device • Devices relying on vertical displacement • Road humps • Bump • A pipe half –buried which can potentially damage the vehicle and cause loss of control

  42. Devices and techniques for managing speed and volume (continued) • TRRL or Watts profile hump • Circular in shape, about 2 to 5-inches high and 10-13 ft in length • Flat-topped or plateau road hump • Similar in effect with circular humps but more user friendly (Figure 13.11 page 349) • Has straight approach and departure ramp (typically 1:10 to 1:15) and a flat top, • Length of flat top varies from 7 to 24 ft, so that vehicle has both axle on plateau • Rumble devices • May be placed transversely across the pavement to alert the driver to approaching hazard • Intersections or pedestrian crossing

  43. Devices and techniques for managing speed and volume (continued) • Devices relying on horizontal displacement • Chicanes • Feature extending into the roadway from the curb which referred to as “build outs” in UK and “curb extensions” in Australia (Figure 13.12 page 351) • Pinch points • Sometimes referred to as “slow points” is a short section of a narrow road • Introduces a sudden change in road configuration (Figure 13.13 page 352) • Restructured parking • Rearrangement of parking can be an effective speed control measure (Figure 14.2 page 371)

  44. Devices and techniques for managing speed and volume (continued) • Gateways • May be used at the entrance to towns to announce the start of the build up environment with devices such as • Pinch points • Changes in surface texture and/or color • Road humps • Signing • Landscaping features • Planting • Street furniture

  45. Implementation of devices • Useful guidelines in implementing devices introduced based on worldwide researches • Network configuration should be such that the amount of traffic volumes in residential streets are in the range of 2000-3000 vpd • Wide, long streets with house frontages have a poor safety record and should be avoided • Network discontinuities can be made to discourage the entry of non-local traffic • Intersections along street within the residential network should be separated by at least 70 ft

  46. Implementation of devices (continued) • Action is usually required when 85 percentile speeds exceed 35 mph • The effect of speed control devices is localized, which introduce the following relationships between 85 percentile speeds and spacing • 18 mph : 250 ft spacing • 24 mph : 330 ft • 27 mph : 500 ft • 36 mph : 670 ft • Chicanes should not be used when traffic volume exceeds 600 vph

  47. Implementation of devices (continued) • Chicanes will constraint vehicle speed to less than 20 mph if it is 30-45 ft long • Chicanes and road humps should only used on straight sections • Rumble devices can be problem for bicyclists and pedestrians • They should not exceed 0.6 inches in height and a gap of about 30 inches be left between the device and curb to allow passage for bicycles • Road humps lower than 2 inches give virtually no slowing effect

  48. Parking • An important task for traffic management is how and where to store the vehicles since they are spending the majority of their time at rest • Parked or parking vehicles are particularly associated with pedestrian fatalities • Ross Silcock Partnership suggest that planning for parking involves consideration of three factors • The need to maximize access to traffic generating facilities • The need to minimize interruption to moving traffic • The need to minimize traffic accidents

  49. One-way streets • One-way streets tend to be inherently safer than two way streets • Friction from an opposing traffic stream has been removed • Generally one-way operation leads to • Higher speeds and longer trips • Fewer stops • Traffic flow tends to be more orderly • More confusing for pedestrians, but on the other hand conflicts at intersections are reduced • Pedestrian accidents and accidents at low volume unsignalized intersections are particular problems

  50. Truck routes • Control of trucks through truck routing is a means of implementing an urban traffic management scheme • Its application is to pursue amenity objectives, not safety objectives • The only reference from the safety stand point is to trucks routing of hazardous materials • Certain ideal guidelines from a number of studies to address the routing for trucks which are carrying hazardous materials are as follow: • All freeways and control access facilities are likely to be suitable