One question I get asked a lot when training is “Can you give us some tips for designing roundabouts for capacity without compromising safety?” My normal response is that each project is different and that each site has it’s own set of challenges, one of the biggest is normally that the challenges generally unravel as the projects goes on rather than being presented up front and in one complete list. But it occurred to me recently, when demonstrating AutoTrack Junctions, that there is a specific reason behind each and every grip and they’re very much based around these tips. So below is a pretty complete and detailed roundabout model as I wanted to show how to overcome common challenges if they arise late in the project and you’re model is already very complete. I’ve laid out 5 tips on how to tackle common design issues underneath.
1. Over-saturated approaches – If you need to alleviate a predicted or recognized issue with capacity, you don’t just want to increase road space too much as this can lead to excessive speeds and safety issues. Roundabouts are in no way a series of linked T-Intersections, they’re a single entity and so the laws of cause and effect very much apply – for example a lack of gaps on one approach for entering vehicles could actually be improved by sacrificing capacity at the previous entry; therefore creating more gaps at the target entry (this is assuming that sufficient capacity reserve is held at the previous entry but it’s a good example of how the intersection as a whole can be balanced between approaches in a very logical manor); think about a signalized intersection and how you would divide the available green periods out of the cycle time amongst the different approaches – a queue building on one approach means another approach is likely to be getting a green signal to go. One of the most critical geometric dimensions in dictating the capacity of a roundabout approach is the overall, effective approach width at that entry. However, it’s common that either a potential increase in vehicle entry speed, right of way constrictions or increased material/ construction costs rule out much of a change here. So, effective flare length (the effective but not overall [and so often surprisingly short] length over which the widening of the approach/ entry section develops) then becomes a designers best friend. Increasing this normally provides a relative increase in capacity without greatly altering entry width, vehicle trajectory/ speed or construction cost.
2. Balancing road alignments and vehicle speeds – Contrary to bad advice, there’s no need to get the placement of the circle right at the beginning, you won’t know at that point how your design is going to evolve and as intimidating as it might sound, moving the centre of the whole intersection (the circle in the case of a roundabout) can actually be a lot less intensive, time wise, than trying to fix one approach at a time. Remember; the onward chain effect at roundabouts has no start or end point as a circle has no mathematical start or end points. In the real world, this means that capacity and speed at each approach can effect onward roundabout entries, then they will in turn effect their onward entries until a theoretically endless relationship is established. So look at the big picture and pick up and drag the circle around to see how the intersecting alignments, vehicle trajectories, approaches and entries would be affected. You may not be likely to cure all approaches this way, but you can quickly get a feel for where the worst problem areas exist and it can be a good way to swap a big problem for much smaller manageable problems. Remember, it’s still cheaper to change at this stage than when the thing is built.
3. Roundabout entry path – There are a number of different potential issues that need to be accommodated at roundabout entries making it one of the most fundamental areas to the design and an area that you’ll probably spend the most time on. Capacity safety and turning movements are all very much affected by the quality of the entry design. In terms of capacity, a wider entry would generally accept a higher capacity up to a point as the human factors create confidence for the driver and vehicles tend to travel faster, accepting smaller gaps; this of course does not make for a safe entry and so capacity and safety often have conflicting needs that need to be balanced (think about the situation at a toll booth – when approaching the booths there is a pressure to get in lane and slow down, being mindful of other vehicles around you; but after getting through the toll booth there is a mass of asphalt to use and it’s more comfortable to speed up quickly). However, it’s also important to ensure that the entry trajectory of vehicles that have lined up, ready to enter the roundabout, is feeding them into the direction of their target lane. Too often poor road markings direct vehicles either dangerously straight into the centre island or into vehicles in a neighbouring lane. If possible, it’s best to slow traffic down “calmly” and efficiently rather than with any sudden and abrupt changes in speed or direction.
4. Turning movements – The biggest problems for large vehicles are often thought of as being inside the circle, but if you have any experience in the trucking industry you’ll know that lining up and positioning in advance is the key. Therefor, the issue is more often than not actually located in the circle, but actually a lack of space or poor alignment on approach and entry to allow the driver of a heavy load to line up their vehicle in advance. The problems may show up in the circle but think about how common it is to see tyre tracks on grass verges and broken kerbstones on entry and exit (this is because the driver knows they won’t fit). If a vehicle has a large inside turning radius then it’s going to need to be positioned out wide before it starts to make the turn, also the farthest point to the inside of the vehicle may well be a body section of a trailer so think about the height of anything that it may end up hitting, or better yet model it. Another often overlooked note is that more and more larger vehicles are on our roads, especially those linked to the energy sector, and these often have complicated extendible trailers of secondary steering that make them a lot more manoeuvrable; but as we try to design compact and efficient roundabouts be sure to also check for the larger rigid body vehicles that will travel through the roundabout as these often have a larger turning circle and are slightly less manoeuvrable than articulated vehicles when travelling forward.
5. Entries and exits belong to the same road – It’s very true that if designing a rural roundabout with higher speeds and no pedestrian or cyclists likely to be present, then the ideal option may be to design a “flatter” exit that expedites traffic from the circle as efficiently as possible. This can also allow more room to add the entry deflection that would be necessary with a higher speed approach. If however the roundabout were in an urban location or had pedestrian crossings present a flat exit would allow a higher exit speed, causing safety risk to pedestrians and causing unnecessary land-take. Either way, it’s a common mistake to treat the exit and entry lanes as separate roads; they each have different jobs to do in terms of their ultimate function but they share the over-arching responsibility of safely connecting a road and the actual roundabout circle and to separate them with too much width in between could cause quite an increase in construction cost and also a much longer overall distance for pedestrians to cross. Aligning roundabout approaches against an alignment that is offset toward the exit side from the existing road alignment can often give a decent starting point by naturally adding entry deflection, removing the symmetry between entry and exit and still allowing a small enough exit radius; but of course this depend on the existing road alignments and will not necessarily work in extreme cases.