A common conflict that is important to consider particularly at signalised intersections is between people wishing to cycle straight ahead and motorists wishing to turn left; at some point the paths of these two user groups must cross. There are a range of ways his conflict can be accommodated at signalised intersections. Each design has implications for cyclists using the intersection.
The first three diagrams below show the location of the different conflict points for slip lanes, exclusive left turn, and shared through and left turn arrangements, respectively. People on bikes would most commonly be provided with conventional cycle lanes in these three layouts, but it is also possible to achieve some form of physical separation (eg with flexiposts). A fourth diagram shows a kerbside cycle facility to the left of an exclusive left turn lane, and this layout would only ever occur with a physically separated facility. Further detail on each of these four layout types is given in sub-sections below.
Slip lanes and exclusive left-turn lanes are safer than shared through and left lanes for cyclists (from research for Austroads titled Effectiveness and selection of treatments for cyclists at signalised intersections(external link), (Hughes, 2014), based on the data from the Austroads research found that it is on average four times safer to resolve this conflict in a midblock location compared to a location at the limit line of an intersection. This is because there is less demand on the cognitive ability of drivers at the midblock, i.e. they can make a decision based on what is happening ahead of them, rather than having to observe what happens to the left of their vehicle or behind them once they reach the limit line.
The Austroads research (external link)did not distinguish between layouts with slip lanes and layouts with exclusive left turn lanes in terms of risk to people cycling. However, the critical risk factor for these two layouts is the speed differential between the conflicting movements. In general, the greater the distance of the conflict point from the limit line or the less physical constraint is imposed by a slip lane layout, the higher the speed differential between a motor vehicle and a cyclist. A higher speed differential usually results in less safe conditions for cyclists.
The Austroads research also did not study protected cycle facilities at signalised intersections, or situations where exclusive phasing is provided for cyclists which involve temporal separation of cyclists from other road users; because the two surveyed cities (Adelaide, Australia and Christchurch, New Zealand) did not have infrastructure or signal operation of this type at the time. Protected cycle facilities and temporal separation have different implications to filter turning (e.g. from shared through and left turning lanes), and the relative safety outcomes in the New Zealand context are currently not known.
Wherever a facility that provides for cyclists travelling straight ahead is situated to the left of a traffic lane that provides for left turning vehicles (i.e. an exclusive left turn lane, or a shared through and left turn lane), careful consideration must be given to the signal phasing.
A left-turn slip lane is a lane separated from an adjacent lane by a marked or raised triangular island. This results in the conflict between left-turning motorists and cyclists travelling straight ahead occurring prior to the intersection limit line. There are two fundamentally different slip lane designs:
free-flow slip lane characterised by an exclusive merge/acceleration lane
high entry angle slip lane characterised by the lack of an exclusive merge/acceleration lane.
There are also many older slip lanes of low entry angle type without an exclusive merge/acceleration lane to turn into; this is no longer considered best practice.
If certain slip lanes on cycle routes are criticised, this is generally in relation to the speed at which vehicles are able to turn off the through route (across the cycle facility) and the length of the conflict area that cyclists are exposed to. The lower the speed differential, the safer the situation for people on bikes. The available tools for minimising the speed differential are to use high entry angle slip lanes, and to reduce the length of the auxiliary (left turn) lane prior to the slip lane (note that there may not be an auxiliary lane at all). Other useful speed control measures are raised platforms within the slip lane at the pedestrian crossing point, possibly with a formal zebra crossing installed.
The intended user types must be considered when developing an appropriate design for cycling across a slip lane. The diagrams below show two options for crossing a slip lane: the first arrangement involves continuing the cycle provision directly across the slip lane, the second option is to provide a cycle facility off-road across the slip lane. Note the latter arrangement is the only viable option for bi-directional facilities because contra-flow cycling is only permitted in physically separated facilities and this could not be achieved other than crossing a slip lane where pedestrians cross it.
Less confident cyclists may not want to travel directly across the slip lane entrance and may choose not to use such a facility. Conversely, more confident cyclists may not want to travel on a facility that deviates and takes longer to cross the slip lane.
Often cycle routes or facilities are retrofitted through intersections with existing slip lanes. The table below includes treatments that should be considered to minimise risks to cyclists travelling past slip lanes.
Table: Possible treatments at slip lanes
High entry angle slip lane
Slip lanes that intersect the cross road at a high angle and do not provide a merge/acceleration lane assist with slowing turning traffic. This also assists with improving safety for cyclists travelling on the cross road.
Platform in slip lane to manage speeds and accommodate pedestrian crossing
Installing a platform in the slip lane assists with reducing vehicles speeds. Platforms also assist with improving safety for pedestrians crossing the slip lane and are often combined with zebra crossings.
Where the slip lane forms part of a bus route longer ramps should be considered to increase comfort for bus passengers.
Reduce length of deceleration lane/ provide no deceleration lane
Reducing the length of the deceleration lane on the approach to the slip lane reduces the length of road cyclists must travel between two lanes of moving traffic. This is also likely to reduce speeds on the approach to the intersection, ie left-turning vehicles need to travel in the main approach lane for longer before slowing to turn. This has the positive knock-on effect of reducing the speed of through traffic.
It is noted in this example, the entry to the slip lane is far from the limit line and the geometry allows drivers to enter the slip lane at higher speeds.
Coloured surfacing across conflict area
Safety performance is improved where coloured cycle lanes are marked across the transition (39% reduction in crashes according to Turner et al (2011)). Green coloured surfacing should be used across all conflict areas.
Ultimately, the TCD Manual Part 4 will provide more information on coloured surfacing at intersections.
Consideration should also be given to cyclists travelling past the slip lane exit on the cross road (as shown in the diagram below). Slowing the speed of vehicles turning out of the slip lane will improve safety for these cyclists, and for other road users.
General design guidance for left turn slip lanes as well as the signage and marking requirements will be provided in the TCD Manual Part 4. In the interim, guidance on markings for cycle lanes adjacent and across a slip lane is provided in MOTSAM Part 2 Section 3 Figure 3.38.
Only protected cycle facilities should be located to the left of an exclusive left-turn lane. Operational issues of such an arrangement are discussed under signal phasing.
An exclusive left-turn lane must be located on the kerbside (left) of a cycle lane where one is provided. Without physical protection, motorists can turn into an exclusive left-turn lane over the entire length of the lane and therefore cyclists are exposed to conflict over this length. Furthermore, as the length of an exclusive left turn lane increases, motorists can enter the lane further away from the limit line and are therefore able to travel at higher speeds, ie there is risk of conflict at a point of high speed differential. Therefore, minimising the length of an exclusive left-turn lane will minimise risk to cyclists. This applies both to layouts with cycle lanes, as well as where cyclists travel in mixed traffic. An example of exclusive left-turn lanes with cycle lanes is shown below.
Shared through and left lanes on intersection approaches pose a higher risk to cyclists as they generally travel on the left of these lanes and are therefore in the path of turning traffic. A cycle lane adjacent to a shared through and left lane results in motorists and cyclists conflicting at the limit line. This can be confusing for both motorists and cyclists as to who has right of way, and it is easy for drivers to overlook a person on their left. Truck drivers, in particular, have blind spots in which they simply cannot see a person riding a bicycle to their left.
Narrow separators and flexi-posts have been trialled to discourage vehicles from encroaching into the cycle lane. While this may not pose a strong physical barrier to motorists, it does offer protection to cyclists by making facilities more conspicuous. This treatment can be applied for short lengths of cycle lane at intersection approaches, effectively making it into a protected cycleway as the cycle lane ceases to be physically available to general traffic (ie part of the roadway).