Intersection Safety S. M. SOHEL MAHMUD Assistant Professor Accident Research Institute (ARI), Bangladesh University of Engineering and Technology (BUET) Dhaka-1000, Bangladesh 1 Outline of the Presentation Basic elements n intersection design Type and selection of intersection type Designing Safe Intersection: Principles and issues Conflict minimization Speed reduction Other safety issues at intersection design 2 INTERSECTIONS = INTERRUPTED FACILITIES Definitions and key elements INTERSECTIONS An intersection is defined as an area where two or more roadways join or cross. Each roadway extending from the intersection is referred to as a leg. Theintersection of two roadways has usually four legs (or three if there one of the roadway is ended). The leg used by traffic approaching the intersection is the approach leg, and that used by traffic leaving is the departure leg. The major street is typically the intersecting street with greater traffic volume, larger crosssection, and higher functional class. The minor street is the intersecting street likely to have less traffic volume, smaller crosssection and lower functional classification than the major street. Intersection Safety Intersections are the most critical element of the road network At grade intersections are risky Because different road users (vehicles, pedestrians, cyclists) are required to use the same space Collision is only avoided if they are separated in time/space In US over one-half of reported urban crashes and over onethird of reported rural crashes are at intersections In Australia 43% of urban crashes and 11% of rural crashes are at intersections Channelization is the separation or regulation of conflicting traffic movements into definite paths of travel by traffic islands or pavement markings (regulation of traffic). 3 4 Intersection Safety (continued) The main factors affecting safety at intersections Number of legs Angle of intersection Sight distance Alignment Auxiliary lanes Channelization friction Turning radii Lighting Lane and shoulder widths Right of way (rules, signs, signals) Approach speed Driveways Five basic elements should be considered in intersection design. 1. Human Factors 2. Traffic considerations 3. Physical elements 4. Economic Factors 5. Functional Intersection Area Human Factors Driving habits Ability of drivers to make decisions Driver expectancy Decision and reaction time Conformance to natural paths of movement Pedestrian use and habits Bicycle traffic use and habits 5 6 1
Five basic elements should be considered in intersection design. Five basic elements should be considered in intersection design. 1. Human Factors 2. Traffic considerations 3. Physical elements 4. Economic Factors 5. Functional Intersection Area Traffic considerations Design and actual capacity Design-hour turning movements Size and operating characteristics of vehicle Varity of movements (diverging, merging, weaving, and crossing) Vehicle speeds Transit involvement Crash experience Bicycle movements 7 Pedestrian s movements 1. Human Factors 2. Traffic considerations 3. Physical elements 4. Economic Factors 5. Functional Intersection Area Physical elements Character and use of abutting property Vertical alignment at the intersection Sight distance Angle of the intersection Conflict area Diverging and merging lanes Geometric design features Traffic Control devices Lighting equipment Safety features Bicycle traffic Environmental factors 8 Cross walks Five basic elements should be considered in intersection design. Five basic elements should be considered in intersection design. 1. Human Factors 2. Traffic considerations 3. Physical elements 4. Economic Factors 5. Functional Intersection Area Economic Factors Cost of improvements Effects of controlling or limiting right-of-way on abutting residential or commercial properties where channelization restricts or prohibits vehicular movements Energy consumption 1. Human Factors 2. Traffic considerations 3. Physical elements 4. Economic Factors 5. Functional Intersection Area 9 10 Intersection Types Intersection Types Road Geometry T-intersections, Y-intersections, Cross intersections, Multi-legged junctions and Traffic Circle: include roundabouts, Grade separation Traffic Controls Uncontrolled intersections, Controlled Intersection Signal-controlled Yield-controlled intersections Stop-controlled intersections Box Junction Road Geometry T-intersections, Y-intersections, Cross intersections, Multi-legged junctions and Traffic Circle: include roundabouts, 11 12 2
Intersection Types Traffic Controls Uncontrolled intersections, Controlled Intersection Signal-controlled Yield-controlled intersections Stop-controlled intersections Box Junction 13 14 Selection of Intersection REMEMBER THAT 1. Intersections are more complicated areas for drivers than uninterrupted facilities. Drivers have to make split second decisions within intersections by considering their routes, intersection geometry, speeds and directions of other vehicles etc. A small error in judgment can cause accidents. Understanding this is central to intersection designs and to determine capacity. 2. Main function of intersections is to provide change of direction. Direction changes within intersections define conflict points 15 3. Intersections are sources of congestion in urban areas. Maneuvers within intersections cause delays. 16 Designing Safe Intersection Four basic rules of good design to reduce the likelihood of traffic accidents include: Designing Safe Intersection Four basic rules of good design to reduce the likelihood of traffic accidents include: 1. Reducing and Separating the Points of Conflict 2. Keeping it Simple 3. Minimizing the Area of Conflict 4. Controlling Speed 1. Control of Points of Conflict 2. Keeping it Simple 3. Controlling Speed 17 18 3
Basic types of maneuvers/ conflict within intersections -Crossing -Merging -Diverging -Weaving Basic types of conflicts/manoeuvres within intersections Crossings may be direct, if the angle of skew is between 75 and 105 degrees, or oblique if the angle is in the range of below 75 or above 105 degrees. (Oblique skews should be voided if at all possible). Diverging is a traffic operation when the vehicles moving in one direction is separated into different streams according to their destinations. Merging is the opposite of diverging. Merging is referred to as the process of joining the traffic coming from different approaches and going to a common destination into a single stream. Weaving is the combined movement of both merging and diverging movements in the same direction. All maneuvers within intersection result in conflicts 19 20 Conflicting Points of Different Type of Intersections 21 22 Merging Diverging 23 24 4
Merging Diverging =4 =4 Total = 8 25 26 Crossing Merging Diverging Crossing Merging Diverging =3 =3 =3 Total = 9 27 28 Crossing Merging Diverging 29 30 5
Crossing Merging Diverging =16 =8 =8 Total = 32 Number of Legs and Conflicts Times 3 8 16 28 31 32 Control of conflicts 1. Minimizing the number of conflict points; 2. Separating points of conflict; 3. Minimizing the area of conflict; Control of conflicts 1. Minimizing the number of conflict points by prohibiting certain traffic movements at an intersection (any right turn restriction) by conversing conventional intersection to a roundabout 33 34 Minimizing the number of conflict points Minimizing the number of conflict points Prohibiting certain traffic movements at an intersection Turning restriction U-loop for prohibiting right turn 35 36 6
Minimizing the number of conflict points Conversion conventional intersection to a roundabout 2. Separating points of conflict By using two separated intersection instead of a single more complicated one By the addition of deceleration lanes, realignment of the intersection. By giving preference to major movements By discouraging undesirable traffic movements 37 38 2. Separating points of conflict by using two separated intersection instead of a single more complicated one 2. Separating points of conflict Points of conflict can be separated/reduced by the addition of deceleration lanes, realignment of the intersection. 39 40 RHS Staggered Junction (Two T-junction with in 60 m) LHS RHS Staggered Junction (Two T-junction with in 60 m) LHS Which one Safer??? Which one Safer??? 41 42 7
2. Separating points of conflict Giving preference to major movements Separating the traffic streams into auxiliary lanes reduces the number and severity of rear end crashes and increase capacity 43 44 2. Separating points of conflict Discourage Undesirable Traffic Movements Traffic islands and corner radii can be used to discourage motorists from taking undesirable travel paths, and encourage them to take defined ones. 3. Minimizing the area of conflict; Large uncontrolled carriageway areas within intersection provide greater opportunities for collisions resulting from unexpected vehicle maneuvers To prevent this traffic island channelisation can be used. 45 46 Designing Safe Intersection Four basic rules of good design to reduce the likelihood of traffic accidents include: 1. Reducing and Separating the Points of Conflict 2. Keeping it Simple 3. Minimizing the Area of Conflict 4. Controlling Speed 47 Keeping it Simple Complicated intersections have poor accident records. A fundamental check is to imagine what a driver, using the intersection for the first time, would do. Two requirements are paramount: No driver should need special knowledge of how to negotiate the intersection. There should be a clear order of precedence within the intersection. 48 8
Traffic Streams Should Merge/Diverge At Flat Angles And Cross At Right Angles Merging / Diverging: Keeping it Simple Merging / diverging should be as flat as possible (max 15 0 ). Crossing: Crossings should be direct (90 0 ). The angle of skew range have to be between 75 0 and 105 0. Oblique angles should be avoided if it is possible, or redesign according to figures a d: 49 The angle of intersection is formed by the intersecting streets centerlines (best angle is between 75 and 105 ACCIDENT degrees). RESEARCH INSTITUTE 50 Keeping it Simple Control Access in the Vicinity of an Intersection Driveways (i.e. approaches to estates, parking places) should not be designed within the function area of a newly designed intersection. If such access points already exists and closure is not possible for practical reasons, than channellisation techniques should be used to prevent entering vehicles from crossing the traffic flow, i.e. the vehicles entering the intersection from the driveway should always merge with the nearside traffic stream. Designing Safe Intersection Four basic rules of good design to reduce the likelihood of traffic accidents include: 1. Reducing and Separating the Points of Conflict 2. Keeping it Simple 3. Minimizing the Area of Conflict 4. Controlling Speed 51 52 Y- Intersection 70 0 to 110 0 70 0 to 110 0 Major conflict Minor conflict T- Intersection 70 0 to 110 0 Skewed Oblique or Offset Cross Minimizing the area of conflict is achieved by reducing the area of pavement where conflict can occur by defining vehicle paths Channelization Roundabout Multi-Legged 53 54 9
Designing Safe Intersection Four basic rules of good design to reduce the likelihood of traffic accidents include: 1. Reducing and Separating the Points of Conflict 2. Keeping it Simple 3. Minimizing the Area of Conflict 4. Controlling Speed Controlling Speed Control of speed The speed of vehicles through an intersection depends on: Alignment; Road environment; Traffic volume and composition; and Traffic control devices. 55 56 Relative speed The safety of an intersection depends largely on achieving low relative speeds. Relative speed is the vectorial speed of convergence of the vehicles in a conflict maneuver. Controlling Speed Low relative speed conditions at intersections can be obtained by: Choosing a layout where conflicting movements cross at angles less than or equal to 90 degrees; Providing a layout or alignment that slows down approaching vehicles; and Providing deceleration lanes. Controlling Speed 57 58 Controlling Speed Controlling Speed Relative speed Minimizing relative speed of conflicting movements. Minor road vehicles should approach the intersection slowly. Than they can easily stop and give way to through traffic. Use roundabout or traffic island This method also has the advantage that it prevents overtaking in closer area of the intersection. 59 60 10
Controlling Speed Improvements in approach and relative speed due to realignment and channelization Other issues need to be considered 1. Provide Refuges for Vulnerable Road Users 2. Provide Reference Markers for Road Users 3. Provide Advance Warning of Change and Good Safe Locations for the Installation of Traffic Control Devices 4. Provide Safety Stopping Sight Distances and Keep The Sight Triangle Without Any Obstructions 61 62 63 64 Provide safety stopping sight distances and keep the sight triangle without any obstructions Hazardous Satisfactory Figure 4.4: Satisfactory and hazardous sight distance 65 66 11
67 68 Lighting 69 Stop or yield sign 70 Channelization Let me show some pictures for other safety issues at intersection 71 72 12
Thank you all Ensure Sustainable Safety by Design and Planning 73 13