GEOMETRIC DESIGN OF ROUNDABOUTS AND TRAFFIC S IGNAL SIGNAL CONTROLLED

Transcription

1 TD 16/07 and TD 50/04 GEOMETRIC DESIGN OF ROUNDABOUTS AND TRAFFIC SIGNAL CONTROLLED JUNCTIONS DMRB Vol 6 File 6 0

2 ROUNDABOUTS: INTRODUCTION Principles Roundabout Types Entry Deflection Capacity Visibility Points to Note in Design Small Roundabouts Mini Roundabouts Pedestrians and Cyclists 1

3 ROUNDABOUT DESIGN: PRINCIPLES The principle objective of roundabout design is to secure a safe interchange between crossing traffic streams with minimum delay There are two broad regimes of roundabout operation: urban areas: high peak flows, marked tidal variations, physical restrictions on space available rural areas: high approach speeds, low tidal variation, few physical constraints The greatest influence on capacity is entry width, which often needs to be greater in urban situations The greatest influence on safety is entry deflection, which is aimed at ensuring that vehicles cannot enter the circulatory carriageway faster than 50kph 2

4 ROUNDABOUT TYPES There are two main types of roundabout: normal roundabouts, which include a kerbed central island 4m or more in diameter and has 2 or more entry lanes mini roundabouts, which include a flush or slightly raised central island 4m or less in diameter A third type, a Compact Roundabout, has single lane entries and exits and used where the speed limit is <60kph 3

5 NORMAL ROUNDABOUTS: ENTRY DEFLECTION Entry deflection is one of the greatest influences on safety at normal roundabouts The vehicle entry path for each approach is required to include a length of 20-25m, within the 50m before the yield line, which is on a radius of 100m or less The vehicle path is intended to represent the racing driver s line as it assumes that: there is no other traffic around the vehicle will be positioned on the approach such that the roundabout can be negotiated at maximum speed the driver will ignore lane markings, but will not enter the opposing traffic lane 4

6 ENTRY DEFLECTION: EXAMPLES Note: The path should commence 1m from the nearside kerb or 1m from the centreline marking, whichever gives the fastest path The green path is not appropriate, although it may appear to be, as it follows the nearside channel for a longer distance 5

7 ENTRY DEFLECTION: EXAMPLES Note: The 100m radius can occur well in advance of the yield line when the approach is on a left hand curve 6

8 ENTRY DEFLECTION: EXAMPLES Note: The requirement to include paths commencing 1m from the centreline marking makes it difficult to achieve deflection when the approach is on a right hand curve 1m min 1m min 1m min 7

9 ENTRY DEFLECTION: WAYS TO ACHIEVE A good method for creating entry deflection is to stagger the arms This is consistent with provision of the required easy exits It is not good practice to generate entry deflection by sharply deviating the approach roads to the right close to the roundabout and then to the left at entry 8

10 NORMAL ROUNDABOUTS: CAPACITY Roundabout Capacity is assessed using ARCADY Six geometric parameters are input for each approach: Approach half width: v Entry width: e Effective flare length: l Entry radius: r Entry angle: Φ Inscribed circle diameter: D 9

11 NORMAL ROUNDABOUTS: CAPACITY Although six geometric parameters are input for each approach, entry width and effective flare length have by far the greatest effect, because: additional entry width provides more lanes at the yield line and thus enables more vehicles to accept a given gap in the circulating flow additional flare length provides a greater capacity for the feed in to the front row at the yield line The general aim in design should be to achieve a Ratio of Flow to Capacity (RFC) value of 0.85 on each approach This reflects the +/- 15% tolerance inherent in the entry capacity formulae, due to site to site variation 10

12 ROUNDABOUT DESIGN: VISIBILITY Roundabout specific requirements for visibility include: visibility to the right at entry forward visibility at entry circulatory visibility Forward visibility on each approach should be in accordance with NRA TD9 11

13 ROUNDABOUT DESIGN: POINTS TO NOTE The maximum entry widths are: 10.5m for a single carriageway approach 15m for a dual carriageway approach The extra capacity apparently provided by increasing the effective flare length beyond 40m is derived from extrapolation beyond experimental data and should be treated with caution The circulatory carriageway should: be circular in plan have a constant width of between 1.0 and 1.2 times the maximum entry width, subject to a maximum of 15m Approach gradients should be a maximum of 2% 12

14 DESIGN OF SMALL NORMAL ROUNDABOUTS The smallest Inscribed Circle Diameter (ICD) that will accommodate the Design HCV is 28m It is very difficult to achieve entry deflection with ICDs below 40m Compact roundabouts have an ICD range of 28 to 36m

15 DESIGN OF MINI ROUNDABOUTS TD45 Mini roundabouts should only be used when all approaches are subject to a speed limit of 50kph or less They can be extremely useful in improving existing urban junctions that experience safety or side road delay problems The central circular marking should be: 1to4mindiameter domed up to a maximum height of 125mm (for a 4m diameter island) Where physical deflection is not possible, road markings and small traffic islands should be used to induce some deflection 14

16 ROUNDABOUT DESIGN: PEDESTRIANS & CYCLISTS CHAPTER 5 & 6 OF TD 16 Although roundabouts have a good overall safety record, this does not apply to two-wheeled wheeled vehicles Careful consideration should therefore be given to meeting the requirements of pedestrians & cyclists in roundabout design Care should be taken if proposing traffic signal controlled crossings are provided in the vicinity of roundabouts, as drivers could mistake the traffic signals for part of a signal controlled roundabout 15

17 ROUNDABOUT DESIGN: PEDESTRIAN & CYCLISTS Chapter 5 Details Requirements for Pedestrians & Cyclists Chapter 6 Design Hierarchy 16

18 TRAFFIC SIGNAL-CONTROLLED JUNCTIONS 17

19 SIGNAL-CONTROLLED JUNCTIONS: PRINCIPLES for TD 50/04 The principal objective of introducing signal control is to improve operational efficiency, by: reducing congestion and conflict between different vehicle movements within the available road space providing all road users with the maximum degree of safety and convenience Signal control may be considered in association with other strategies, such as: augmenting g or altering a natural route priority providing a safe environment for pedestrians and cyclists providing priority to buses and coaches area traffic control light rapid transit systems 18

20 SIGNAL-CONTROLLED JUNCTIONS: DESIGN SPEED TD50/04 assumes that the 85 th percentile speeds on all approaches to a signal-controlled junction are below 104kph (65mph) Traffic signals are not recommended d where this speed is exceeded d High approach speeds can result in: difficult decisions for drivers when green changes to amber right turning drivers misjudging gaps between oncoming vehicles Where the design speed related parameters cannot be achieved, traffic management measures should be introduced d to reduce the approach speeds to the appropriate value for the available SSD 19

21 SIGNAL-CONTROLLED JUNCTIONS: VISIBILITY Signal-controlled junction intervisibility requirements are not as onerous as those for major/minor priority junctions in TD 42/95 SSD on the junction approaches should be in accordance with NRA TD9/00, subject to the visibility envelope being increased to include the signal head 20

22 SIGNAL-CONTROLLED JUNCTIONS: CAPACITY Signal Controlled Junction Capacity is assessed using: OSCADY or LINSIG (for isolated junctions) TRANSYT or SCOOT (for co-ordinated ordinated network) For OSCADY, the geometric parameters to be input for each approach are: number of lanes lane widths (entry and exit) gradient corner radius flare details The proposed signal staging arrangements are also inputed The general aim in design should be to achieve a Ratio of Flow to Capacity (RFC) value of 0.90 for each traffic stream 21

23 SIGNAL-CONTROLLED JUNCTIONS: GOOD DESIGN PRACTICE The addition of certain design features in signal-controlled junction layouts can positively influence the overall safety of the junction. Examples include: high friction surfacing on the immediate approaches clear and unambiguous traffic signs and road markings removal of right turn movements if feasible road markings to promote non-hooking right turns specific measures for cyclists road lighting types of guardrail that give intervisibility between pedestrians and drivers 22

24 SIGNAL-CONTROLLED JUNCTIONS: PEDESTRIANS & CYCLISTS Specific provision for pedestrians and cyclists should be included wherever possible. This provision may include: Cyclists mandatory cycle lanes, or advisory cycle lanes where space is restricted cycle tracks with combined pedestrian/cyclist crossings advance stop lines for cyclists Pedestrians provision of pedestrian signal phases and pedestrian refuges tactile paving surfaces and dropped kerbs locating crossings on or near to desire lines provision of good pedestrian/driver intervisibility 23

25 SIGNAL-CONTROLLED JUNCTIONS: ROUNDABOUTS Traffic signals can resolve specific traffic problems at roundabouts without the need for geometric changes In addition to increasing overall capacity, benefits may also include: separating the conflict between pedestrian and vehicular movements improving safety (particularly for cyclists) reducing circulatory speeds providing a better balance of queues between the arms 24

26 WORKSHOP: EXERCISE Identify the number of Geometric Departures and Relaxations Local Road with a Design Speed of 60kph. Assume the access at chainage 20 to be a) Access to houses and then b) Access to fields 25

27 WORKSHOP: EXERCISE Example 2 26

28 TD 22/06, TD 39/94 & TD 40/94 GRADE SEPARATED JUNCTION DESIGN DMRB Vol 6 File 7 27

29 GRADE SEPARATED JUNCTIONS INTRODUCTION Standards Types of Grade Separated Junction Standard Grade Separated Junctions: Principles Design of Merges and Diverges Geometric Standards Design of Weaving Sections Compact Grade Separated Junctions Major Interchanges 28

30 GRADE SEPARATED JUNCTIONS: STANDARDS There are 3 basic forms of grade separation They are covered in the following DMRB standards, which are only applicable in Ireland when used in conjunction with the relevant NRA Addenda: TD 22/06 - Layout of Grade Separated Junctions TD 40/94 - Layout of Compact Grade Separated Junctions TD 39/94 - The Design of Major Interchanges 29

31 TYPES OF GRADE SEPARATED JUNCTION 1. Standard Grade Separation Grade separation designed to DMRB Standard TD 22/06 Typical layouts include diamonds, half cloverleaf junctions, dumbell roundabouts and two-bridge roundabouts Layout, traffic signs & road markings reflect the high standard layout, particularly the speeds at which drivers can diverge Junction with Minor Road determined by NRA TD or TD 16 30

32 TYPES OF GRADE SEPARATED JUNCTION 2. Compact Grade Separation Grade separation designed to DMRB Standard TD 40/94 Typical layout consists of compact connector roads linking the mainline to the minor road Justified for much lower traffic flows than would normally be associated with grade separation Reduces environmental impact and landtake Layout, traffic signs & road markings reflect the lower standard layout, particularly the speeds at which drivers can diverge 31

33 TYPES OF GRADE SEPARATED JUNCTION 3. Major Interchanges Grade separation designed to DMRB Standards TD 39/94 and TD 22/06 Generally for intersections between motorways and major roads Provides uninterrupted movements for vehicles moving from one mainline to another by the use of link roads and a succession of diverging and merging manoeuvres Spaghetti Junction 32

34 DIFFERENCE IN SCALE OF TD 22/06 AND TD 40/94 LAYOUTS With low traffic flows, TD 40/94 layouts can provide the advantages of grade separation with much lower cost, land take and environmental impact NB Layouts drawn to same scale 33

35 STANDARD GRADE SEPARATED JUNCTIONS: PRINCIPLES Efficient grade separation should present drivers with the minimum number of unambiguous decision points as they traverse any at grade component of the junction and in merging and diverging Consistent use of signs and road markings should make clear to drivers the lane configurations ahead. Drivers should not be surprised by unexpected features Certain layouts are not recommended due to reduced safety. Examples are: Grade separated junctions on single carriageways Grade separated junctions on dual carriageways within about 500m of the changeover from single carriageway standard Offside merges and diverges 34

36 STANDARD GRADE SEPARATED JUNCTIONS: PRINCIPLES Mixed provision of grade separation and major/minor at grade junctions is not recommended. The consistency of the design for successive junctions is important to avoid driver confusion. Diamond or half-cloverleaf junctions are generally only appropriate for low turning flows Two-bridge roundabouts tend to have large circulatory carriageways which result in high circulating speeds and consequent operational problems. Dumbell roundabout junctions are a preferable alternative 35

37 STANDARD GRADE SEPARATED JUNCTIONS: APPLICATION OF PRINCIPLES MERGES For safety reasons, and to limit interference to mainline traffic, merging traffic should be channelled into the merging area such that it arrives in an orderly manner If joining flows are greater than one lane capacity then an additional lane should normally be added to the mainline as a lane gain Auxiliary lanes which provide continuous merging opportunities should be considered if connector road and mainline flows are close to capacity If the merging flow is greater than the mainline flow, the merging traffic should still give way to mainline traffic, except where lane gains are provided 36

38 STANDARD GRADE SEPARATED JUNCTIONS: APPLICATION OF PRINCIPLES DIVERGES Traffic should be able to leave the mainline as easily and quickly as possible Queuing to rejoin the local network should be prevented from extending back onto the mainline. Auxiliary lanes should be provided if this is likely Auxiliary lanes should also be considered where diverging flows are high, giving diverging drivers a longer time to leave the mainline 37

39 STANDARD GRADE SEPARATED JUNCTIONS: DESIGN OF MERGES & DIVERGES The illustrated Merging Diagram includes: No of lanes on the upstream & downstream mainline and the connector road Lane capacities of 1600 vph for All-Purpose Roads and 1800vph for Motorways It is used to determine the appropriate Merge Layout 38

40 STANDARD GRADE SEPARATED JUNCTIONS: DESIGN OF MERGES & DIVERGES Merge Lane Example Motorway: Merging Flow = 1250vph Mainline Flow = 5600vph Layout B (Parallel Merge) required All Purpose Road: Merging Flow = 1700vph Mainline Flow = 2500vph Layout F (Mainline Lane Gain at Ghost Island Merge) required A similar methodology is adopted for the design of diverges 39

41 STANDARD GRADE SEPARATED JUNCTIONS: DESIGN OF MERGES & DIVERGES TD22/06 Figure 2/4.1 to 5 shows the alternative merge lane layouts In the UK, TD22/92 permits the use of taper merges where the on-slip traffic flow is low. The NRA have chosen not to include such taper merges in Ireland, preferring the parallel merge of Layout B as the lowest standard In rural areas, 2-lane slip roads always require a ghost island merge and, unless traffic flows on the mainline are light, a lane gain on the mainline 40

42 TD 22/06 MERGE LAYOUT OPTIONS Not permitted by the NRA 41

43 TD 22/06 MERGE LAYOUT OPTIONS 42

44 TD 22/06 MERGE LAYOUT OPTIONS 43

45 TD 22/06 MERGE LAYOUT OPTIONS 44

46 TD 22/06 MERGE LAYOUT OPTIONS 45

47 STANDARD GRADE SEPARATED JUNCTIONS: GEOMETRIC STANDARDS Connector Road Types and Design Speeds Interchange Link: a connector road carrying free flowing traffic within an interchange Link Road: a one way connector road adjacent to but separated from the mainline, used where junctions would be too closely spaced Slip Road: a one way connector road between the mainline and the local highway network, which meets the local highway network at grade 46

48 STANDARD GRADE SEPARATED JUNCTIONS: GEOMETRIC STANDARDS Horizontal & Vertical Alignment The horizontal and vertical alignment of the mainline and connector roads at a grade separated junction should be designed to NRA TD9/07. However, studies have shown that the radii of loops (defined as connector roads which pass though an angle of approximately 180º to 270º) can safely be much less than those for curves turning through lesser angles. Adequate warning signs of loops are to be given and clear sight-lines are to be maintained. Thus, minimum loop radii have been reduced to: 75m for loops both on and off a motorway 30m for loops on to an all-purpose road 50m for loops off an all-purpose road 47

49 STANDARD GRADE SEPARATED JUNCTIONS: GEOMETRIC STANDARDS Sight Distance Merges: the SSD shall be related to the design speed of the connector road and shall be maintained up to Des Min SSD upstream of the back of the merge nose. From then on, the SSD shall be that for the design speed of the mainline. Diverges: the SSD related to the mainline design speed shall be maintained until the driver reaches the tip of the diverge nose. SSD can be reduced by one step along the length of the nosing. SSD can then be reduced to that for the design speed of the connector road. 48

50 TD 22/06 FIGURE 4/3A Illustration of Stopping Sight Distance on Merge Slip Road 49

51 TD 22/06 FIGURE 4/3B Illustration of Stopping Sight Distance on Diverge Slip Road 50

52 TD 22/06 FIGURE 4/3C Illustration of Stopping Sight Distance on Slip Road 51

53 GRADE SEPARATED JUNCTIONS: DESIGN OF WEAVING SECTIONS Weaving takes place in the section of carriageway between a merge and a diverge and comprises the intersection of vehicle paths at a small angle Weaving length is measured in a number of different ways, depending on the details of the road layout It is typically the distance between the tip of a merge taper and the tip of the subsequent diverge taper 52

54 GRADE SEPARATED JUNCTIONS: DESIGN OF WEAVING SECTIONS Definition of Terms used in Weaving & Measurement of Weaving Length for Taper & Auxiliary Lane Layouts 53

55 GRADE SEPARATED JUNCTIONS: DESIGN OF WEAVING SECTIONS Rural Motorways and Type 1 Dual c/w National Roads Desirable Minimum Weaving Length = 2km. In extreme cases, where traffic forecasts are at the lower end of the range for the specific carriageway, an Absolute Minimum of 1km can be considered Urban All-Purpose Roads There is no Desirable Minimum Weaving Length. The Absolute Minimum is dependent on the Design Speed and varies between 450m and 100m 54

56 TD 40: COMPACT GRADE SEPARATED JUNCTIONS: PRINCIPAL OBJECTIVES To provide a form of grade separation which can be economically justified at low main line traffic flows Whereas full grade separation to TD 22 can generally be justified at design flows above 30,000 AADT on the main line, compact grade separation to TD 40/94 can generally be justified at design flows above 12,500 AADT on the main line This enables safety to be improved by eliminating right turn manoeuvres between the main line and side roads It also reduces environmental impact of junctions minimises land take improves access across the mainline for pedestrians, cyclists and equestrians 55

57 COMPACT GRADE SEPARATED JUNCTIONS: COMPREHENSIBILITY Drivers on the mainline must be able to comprehend the standard of grade separation that has been provided, so that they do not leave the mainline at such a speed that the left in/left out junction bellmouth cannot be safely negotiated. t The following features help to inform the driver that compact grade separation has been provided: signing to indicate the junction as a priority T-junction rather than a slip road shorter deceleration tapers no count down markers on the junction approach no chevron - hatched noses a large traffic island at the end of any deceleration taper 56

58 COMPACT GRADE SEPARATED JUNCTIONS: GEOMETRIC PARAMETERS Compact grade separated junctions permit the lowest geometric parameters for the design of road layouts anywhere in the NRA DMRB NB Layouts drawn to same scale 57

59 COMPACT GRADE SEPARATED JUNCTIONS: GEOMETRIC PARAMETERS The geometric standards in NRA TD9 provide for design speeds down to a minimum of 50kph. Although TD40/94 utilises the principles contained in TD9, the standards for compact connector roads are based on a design speed of 30kph - 3D Design Speed steps below the current minimum Table 6/1 of TD40/94 contains the main geometric parameters for the design of compact connector roads. 58

60 COMPACT GRADE SEPARATED JUNCTIONS: GEOMETRIC PARAMETERS SSD on Compact Connectors can be reduced to values based on a Design Speed of 30kph: Desirable Minimum: 33m 1 step Relaxation: 26m Gradient Standards for Compact Connector Roads are: Desirable Maximum: 8% 1 step Relaxation : 10% These minimal geometric parameters can provide a useful benchmark when designing layouts in highly constrained urban areas 59

61 NRA TD 41-42: 42: COMPACT GRADE SEPARATED JUNCTIONS GEOMETRIC PARAMETERS Junctions with the mainline should be based on the standards in NRA TD but should include a traffic deflection island similar to the one below, with or without merge/diverge tapers 60

62 COMPACT GRADE SEPARATED JUNCTIONS: TYPICAL LAYOUT OPTIONS 61

63 TD 39: MAJOR INTERCHANGES: PRINCIPLES TD39/94 provides guidance on the design of Major Interchanges, generally as upgrades to existing grade separated junctions Major interchanges provide free-flow flow movements for vehicles moving from one mainline to another by the use of link roads and a succession of diverging and merging manoeuvres The choice of junction location is usually limited compared with completely new construction Traffic Management on existing roads will often influence the assessment of options TD39/94 provides advice on the layouts to be used for junctions, major merges and diverges, and upgrade paths for existing junctions, as well as advice on their operational assessment 62

64 MAJOR INTERCHANGES: LAYOUTS Typical Interchange Layouts: 63

65 MAJOR INTERCHANGES: MAJOR DIVERGES A Major Diverge is defined as a situation where 3 lanes leave the mainline carriageway : 64

66 MAJOR INTERCHANGES: MAJOR MERGES A Major Merge is defined as a situation where 3 lanes join the mainline carriageway : 65

67 MAJOR INTERCHANGES: UPGRADE PATHS TD39/94 provides advice on the improvement of 3 types of existing interchange: 3 level roundabouts 3 leg T interchanges 4 leg interchanges Example: Alternative layouts for the improvement of a 3 level roundabout 66

68 TIME FOR TEA 67