70676 Transport Engineering
CIV3703 Transport Engineering
Source: Austroads
Module 3, Part 3 Geometric design of roads
3.13 Importance of Intersections Intersection: Location at which two traffic streams cross or merge. Capacity and safety – problems
Two basic types: at-grade grade-separated
Grade-separated: higher capacity safer greater cost
Common forms of at-grade Intersections
Problems with At-grade Intersections Vehicle conflicts – accident rates Drainage problems Surrounding land use (shops, etc) Land acquisition costs higher Roadway lighting – require additional pavement area Pedestrian concentrations Priority allocation (eg., directional traffic, pedestrians)
3.14 Types of Conflicting Manoeuvres
Weave
3.15 Types of Intersections [At-Grade] Unchannelised, Unflared Intersections Intersection of minor roads Intersection of minor roads and major roads Priority handled by STOP and GIVE WAY signs.
Unchannelised and Flared Intersections Provide additional capacity for turning traffic.
Channelised Intersections Movement paths delineated by traffic islands.
3.16 Factors Influencing Design Traffic Need to consider all road users – freight, cyclists, peds.
Topography and Environment Alignment, grade, drainage, pub. util., property access.
Economic Considerations Cost benefit analysis
Human Factors Driver behaviour, reaction time, decision time.
3.17 Design Procedure 1.
Obtain traffic data Design hour volumes; through and turning.
2.
Obtain physical data – topography, nearby building, etc.
3.
Obtain road design data – existing & future
4.
Prepare study sketches
5.
Analyse alternatives
6.
Prepare preliminary plans
7. Evaluate preliminary plans Design features; capacity; operational characteristics; overall adaptability; maintenance of traffic during construction; suitability of stage construction.
8. Calculate preliminary cost estimates Including land acquisition; clearing; construction; maintenance; utility changes.
9. Calculate road user costs and benefits 10. Analyse data (7), (8) and (9) to give preferred plan. 11. Prepare final drawings, spec and estimate.
3.18 Principles of Intersection Design Smallest design unit is the individual maneouvre area. Typical road intersection can be considered a combination of a number of elemental maneouvre areas.
Fundamental Principles of Intersection Design
1. Minimise number of conflicting points
Conflict analysis – Case study Before
After
2. Control relative speed
3. Separate points of conflict
Source: Austroads
4. Give priority to major traffic flow
5. Reduce area of conflict
6. Coordinate intersection and traffic control designs
7. Clearly define vehicle paths and conflict areas
8. Control the speed and angle of conflicting traffic movements
Source: Austroads
9. Prohibit undesirable or unnecessary traffic movements
10. Provide adequate capacity
11. Consider the spacing of intersections
12. Consider the needs of all road users
3.19 Geometric Design Standards for At-Grade Intersections
3.19.1 Design Vehicles Provision made for vehicles of legal dimensions, to move with adequate clearance to road furniture and other vehicles. 3 design vehicles used: Design car Design single unit truck, or bus Design semi-trailer
Which design vehicle should be used? Depends on: Location if Intersection Expected traffic composition
Which design vehicle should be used? Turning movements checked with use of turning templates.
3.19.2 Sight Distance Adequate sight distance critical to safe operation. 3 criteria used: Approach sight distance, ASD (minimum – stop); Safe intersection sight distance, SISD (major road traffic observing minor road traffic). Minimum gap sight distance, MGSD
Approach sight distance, ASD Driver height = 1.1 m; object height = 0 ASD =RTV/3.6 + V2/254(d + 0.01a) ASD = approach sight distance (m) RT = reaction time (s) V = operating (85th percentile) speed (km/hr) d = Coefficient of deceleration (also fL) a = Longitudinal grade in %
Safe Intersection Sight Distance (SISD) SISD = DTV/3.6 + V2/254(d + 0.01a)
SISD = Safe intersection sight distance (m) DT = decision time (s) = observation time (3s) + reaction time V = operating speed (km/hr) d = Coefficient of deceleration (also fL) a = Longitudinal grade in % Frequently the minimum sight distance is specified as 2xSD.
Source: Austroads
Example:Minimum gap sight distance, MGSD
Intersection Sight Distances (m) Intersection sight distance based on driver reaction time of 2 seconds
3.19.3 Horizontal & Vertical Alignment Best location for intersections: Straight roads with uniform grade. Where straight horiz. alignment not available: intersection within horiz. curve. Where uniform grade not available: intersections best in sag, not on crest.
3.19.4 Channelisation
Design of channelisation individual for each intersection.
Uses of channelisation Separating conflicting movements; Reducing general area of conflict; Merging traffic streams at small angles; Controlling traffic streams by width; Providing refuge for turning vehicles; Prohibiting certain turns; Improving efficiency and layout of signal systems; Providing protection for pedestrians; Providing installation areas for traffic signals and/or signs.
Source: Austroads
Source: Google maps
3.19.5 Provision for Left-turning Vehicles Simple left turns – appropriate radius. Left turn lanes – signalised intersections, where turning volumes high, or high speeds. Left turn slip lane – left turn lane with corner island. To minimise delays.
3.19.6 Provision for Right-turning Vehicles Auxiliary right turn lanes – used to improve capacity and/or safety.
Length? Sufficient storage (predicted peak hour) Right turn lanes must appear as turn lanes so drivers do not use them as through lanes.
3.19.7. Traffic Lane Widths Ideal: lane widths same as approach roads. Urban situations – often provision made for turning by using narrower lanes. Turning lanes: desirable width at least 3m. Widths as low as 2.4m have been used.
3.20 Roundabouts A channelised intersection in which all traffic through the intersection circulates clockwise around a central island. Entering traffic required to give way to traffic circulating on roundabout.
3.21 Grade Separations and Interchanges Grade-separated intersection: intersecting roads separated in level to eliminate crossing conflict. If intersection also allows turning movements from one road to another, then the intersection is an interchange.
Advantages and Disadvantages Advantages: Capacity approaches that of normal road section; Increased safety and driver comfort; Design is flexible – adaptable to most situations; Staged development often possible.
Disadvantages: Costly; Large land area required; May be confusing to drivers.
Standard Interchange Configurations T or Trumpet (3 leg) Y (3 leg) Cloverleaf Partial cloverleaf Diamond Rotary
T or Trumpet
Y
Cloverleaf
Source: Google
Partial Cloverleaf
Diamond
Source: Google
Rotary
Hi-5 Interchange construction, Dallas, Texas, USA
Source: Google
End of Module 3
Source: Austroads
Module 3, Part 3 Geometric design of roads
3.13 Importance of Intersections Intersection: Location at which two traffic streams cross or merge. Capacity and safety – problems
Two basic types: at-grade grade-separated
Grade-separated: higher capacity safer greater cost
Common forms of at-grade Intersections
Problems with At-grade Intersections Vehicle conflicts – accident rates Drainage problems Surrounding land use (shops, etc) Land acquisition costs higher Roadway lighting – require additional pavement area Pedestrian concentrations Priority allocation (eg., directional traffic, pedestrians)
3.14 Types of Conflicting Manoeuvres
Weave
3.15 Types of Intersections [At-Grade] Unchannelised, Unflared Intersections Intersection of minor roads Intersection of minor roads and major roads Priority handled by STOP and GIVE WAY signs.
Unchannelised and Flared Intersections Provide additional capacity for turning traffic.
Channelised Intersections Movement paths delineated by traffic islands.
3.16 Factors Influencing Design Traffic Need to consider all road users – freight, cyclists, peds.
Topography and Environment Alignment, grade, drainage, pub. util., property access.
Economic Considerations Cost benefit analysis
Human Factors Driver behaviour, reaction time, decision time.
3.17 Design Procedure 1.
Obtain traffic data Design hour volumes; through and turning.
2.
Obtain physical data – topography, nearby building, etc.
3.
Obtain road design data – existing & future
4.
Prepare study sketches
5.
Analyse alternatives
6.
Prepare preliminary plans
7. Evaluate preliminary plans Design features; capacity; operational characteristics; overall adaptability; maintenance of traffic during construction; suitability of stage construction.
8. Calculate preliminary cost estimates Including land acquisition; clearing; construction; maintenance; utility changes.
9. Calculate road user costs and benefits 10. Analyse data (7), (8) and (9) to give preferred plan. 11. Prepare final drawings, spec and estimate.
3.18 Principles of Intersection Design Smallest design unit is the individual maneouvre area. Typical road intersection can be considered a combination of a number of elemental maneouvre areas.
Fundamental Principles of Intersection Design
1. Minimise number of conflicting points
Conflict analysis – Case study Before
After
2. Control relative speed
3. Separate points of conflict
Source: Austroads
4. Give priority to major traffic flow
5. Reduce area of conflict
6. Coordinate intersection and traffic control designs
7. Clearly define vehicle paths and conflict areas
8. Control the speed and angle of conflicting traffic movements
Source: Austroads
9. Prohibit undesirable or unnecessary traffic movements
10. Provide adequate capacity
11. Consider the spacing of intersections
12. Consider the needs of all road users
3.19 Geometric Design Standards for At-Grade Intersections
3.19.1 Design Vehicles Provision made for vehicles of legal dimensions, to move with adequate clearance to road furniture and other vehicles. 3 design vehicles used: Design car Design single unit truck, or bus Design semi-trailer
Which design vehicle should be used? Depends on: Location if Intersection Expected traffic composition
Which design vehicle should be used? Turning movements checked with use of turning templates.
3.19.2 Sight Distance Adequate sight distance critical to safe operation. 3 criteria used: Approach sight distance, ASD (minimum – stop); Safe intersection sight distance, SISD (major road traffic observing minor road traffic). Minimum gap sight distance, MGSD
Approach sight distance, ASD Driver height = 1.1 m; object height = 0 ASD =RTV/3.6 + V2/254(d + 0.01a) ASD = approach sight distance (m) RT = reaction time (s) V = operating (85th percentile) speed (km/hr) d = Coefficient of deceleration (also fL) a = Longitudinal grade in %
Safe Intersection Sight Distance (SISD) SISD = DTV/3.6 + V2/254(d + 0.01a)
SISD = Safe intersection sight distance (m) DT = decision time (s) = observation time (3s) + reaction time V = operating speed (km/hr) d = Coefficient of deceleration (also fL) a = Longitudinal grade in % Frequently the minimum sight distance is specified as 2xSD.
Source: Austroads
Example:Minimum gap sight distance, MGSD
Intersection Sight Distances (m) Intersection sight distance based on driver reaction time of 2 seconds
3.19.3 Horizontal & Vertical Alignment Best location for intersections: Straight roads with uniform grade. Where straight horiz. alignment not available: intersection within horiz. curve. Where uniform grade not available: intersections best in sag, not on crest.
3.19.4 Channelisation
Design of channelisation individual for each intersection.
Uses of channelisation Separating conflicting movements; Reducing general area of conflict; Merging traffic streams at small angles; Controlling traffic streams by width; Providing refuge for turning vehicles; Prohibiting certain turns; Improving efficiency and layout of signal systems; Providing protection for pedestrians; Providing installation areas for traffic signals and/or signs.
Source: Austroads
Source: Google maps
3.19.5 Provision for Left-turning Vehicles Simple left turns – appropriate radius. Left turn lanes – signalised intersections, where turning volumes high, or high speeds. Left turn slip lane – left turn lane with corner island. To minimise delays.
3.19.6 Provision for Right-turning Vehicles Auxiliary right turn lanes – used to improve capacity and/or safety.
Length? Sufficient storage (predicted peak hour) Right turn lanes must appear as turn lanes so drivers do not use them as through lanes.
3.19.7. Traffic Lane Widths Ideal: lane widths same as approach roads. Urban situations – often provision made for turning by using narrower lanes. Turning lanes: desirable width at least 3m. Widths as low as 2.4m have been used.
3.20 Roundabouts A channelised intersection in which all traffic through the intersection circulates clockwise around a central island. Entering traffic required to give way to traffic circulating on roundabout.
3.21 Grade Separations and Interchanges Grade-separated intersection: intersecting roads separated in level to eliminate crossing conflict. If intersection also allows turning movements from one road to another, then the intersection is an interchange.
Advantages and Disadvantages Advantages: Capacity approaches that of normal road section; Increased safety and driver comfort; Design is flexible – adaptable to most situations; Staged development often possible.
Disadvantages: Costly; Large land area required; May be confusing to drivers.
Standard Interchange Configurations T or Trumpet (3 leg) Y (3 leg) Cloverleaf Partial cloverleaf Diamond Rotary
T or Trumpet
Y
Cloverleaf
Source: Google
Partial Cloverleaf
Diamond
Source: Google
Rotary
Hi-5 Interchange construction, Dallas, Texas, USA
Source: Google
End of Module 3
