Lecture 2 - Roundabouts Flashcards
What is a roundabout?
It is a form of channelised intersection with vehicles directed onto a oneway circulatory road.
What are the different types of roundabout?
- Normal roundabout
- Signalised roundabout
- compact roundabout
- mini roundabout
- double roundabout
- grade separated roundabouts
Information about normal roundabouts?
- Kerbed island which is greater than or equal to 4m in width
- Approach roads are dual or single
Information about mini roundabouts?
- improves the performance of existing junctions
- improves existing intersections in urban areas
- Road markings show pattern of movement
- Has a flush or domed circular island of diameter less than 4 metres
- May or may not have flared approaches
- Capable of being driven over where unavoidable
- Never used on high-speed roads
Information about compact roundabouts?
- Has single lane entries and exits on each arm
- Width of circulatory carriageway does not make it feasible for two cars to pass one other
- Has less capacity compared to Normal Roundabout
- Suitable for accommodating the movement of pedestrians and cyclists
- Flexibility in siting pedestrian crossings (non-flared entries/exits)
Information about double roundabouts?
- Junction contains two roundabouts separated by a short link
- May be Mini, Compact or Normal Roundabouts
- especially useful for improving existing staggered junction where they avoid the need to realign one of the approach roads
- joining two parallel routes separated by a feature such as a river, a railway line or a motorway
- at junctions with more than four entries
Information about grade separated roundabouts?
-At least one approach coming from a road at a different level
- At least one movement of traffic passes through the junction without interruption.
- Other movements brought to one or more roundabouts at a different level
- Used at motorway junctions and dual carriageways
To link underpasses, flyovers and other multiple level intersections
- On high capacity roads and those with high approach speeds
Information about signalised roundabouts?
- Traffic signals on one or more approach roads
- operated full time or part time
- signals installed due to overloading or unbalanced flow at one or more entries.
When are roundabouts most effective?
- high proportions and/or high volumes of right turning traffic
- neither feasible nor required to give priority to traffic from any specific road
- an excessive number of accidents involve crossing or turning action
- Stop or Give Way signs lead to unreasonable delays to minor road traffic
- they are the cause of less overall delay to vehicles than traffic signals
- a notable change in road standard e.g. from a dual to a single carriageway
When are roundabouts not effective?
- insufficient space or unfavourable topography
- traffic flows are unbalanced
- steep downhill approaches
- reversible lanes expected to be used during peak traffic periods (British: Tidal flow)
-substantial volumes of vehicular traffic and also heavily used by cyclists and/or pedestrians with no adequate crossings for vulnerable road users - between traffic-signal controlled intersections which may generate queuing back into the roundabout exits or that are subject to linked control
Selecting a roundabout is dependant on a few factors which are?
- whether the approach roads are single or dual carriageway (or grade separated)
- the approach roads speed limit
- traffic flow levels
- the level of non-motorised user (NMU) flow
- other limitations such as land-take
What are other design considerations that have to be made?
- Left-turn Lanes
- Provision for visually and mobility impaired users
- Cross fall
- Traffic Signs and Road Markings
- Surfacing (skid resistance)
- Yellow bar markings
- Landscaping
- Lighting
