How Big Does the Corner Really Need to Be?

In the right context and with the right parameters, it is possible to design corners to be much more compact while still accommodating large trucks.

As a designer who mainly focuses on the non-motor vehicle elements of a street, one of the first things that catches my eye when I review intersections is how big the corners are. Even though the vehicles being designed for are consistent, there’s a surprising amount of variation in the size of a corner (measured by its radius) between different intersections and across cities.

To me, this means there must be some degree of subjectivity among designers, or preferences at the city level, in selecting how big the corners shall be. Subjectivity means an opportunity for a designer’s bias to affect the design – if the designer is accustomed to providing easy accommodations for large vehicles, they’re more likely to keep doing that. My bias is the opposite: I want to design the smallest corner possible that still safety accommodates the largest vehicle expected at the intersection.

Benefits of Smaller Corners

While large radii are helpful (and in some cases necessary) for accommodating large trucks, they are detrimental to the pedestrian environment. As documented in the white paper that I co-authored for Alta Planning + Design called Corner Design for All Users, large corner radii can lead to:

  • faster passenger vehicle turning speeds, which is associated with lower yielding rates to pedestrians and higher chance of injury or death in the case of a collision
  • longer pedestrian crossing distances and more exposure to traffic

Large corners also take up a lot of real estate at an intersection! Compared to a more compact 5m radius curve, a 15m curve requires an additional 43 square metres of space (the size of an average bachelor apartment!). That’s 43 square metres that could have been used for landscaping, a wider sidewalk, or other pedestrian amenities.

Experimenting With Autoturn

With all of this in mind, I decided to try my hand at one of the most common programs used for determining radius: Autoturn. This software contains a massive library of common vehicle types used all over the world and is used to simulate the turning path of those vehicles in a wide variety of circumstances.

Let’s say I need to accommodate a large transport truck being able to make a turn at a corner. It could be because my both streets are truck routes, or simply because that’s what the local context requires. As my example corner, I’ll use the intersection of two major roadways as shown below. I’ve started with a 5m corner radius and assumed 3.5m wide vehicle lanes for simplicity.

Base Case

I’ll start with the most basic turning path, from curb lane to curb lane, at a speed of 20km/h:

Whoa! That truck takes up a lot of space. The hatched area is how much I would need to “chop off” the corner in order to make enough space for that transport to comfortably make a turn here. I would need to increase the radius from 5m to 15m and widen the receiving roadway to make this work. This amounts to about 132 square metres of space and increases the pedestrian crossing distance by a whopping 8m.

Luckily, a designer has some other tools and techniques available to mitigate this. The above can be considered the worst case or base case scenario. Let’s start manipulating the turn path.

Use All Receiving Lanes

First, what if this large vehicle is permitted to use both of the receiving lanes? This is the easiest concession to make, as most of the time a large truck would be turning on a green light where there are no other conflicting movements. The truck driver can then merge back to the curb lane after completing the turn.

With this one change, the affected corner area is reduced by 63%. I still need a larger 15m corner radius, but the amount of widening needed to the receiving roadway is much less, and the pedestrian crossing is increased by just 4 metres instead of 8.

Oversteer

So far, I’ve been assuming the truck turns at 20 km/h. This might be alright in an industrial area, but in an urban area this is much too fast. I’m going to lower the turning speed to 5 km/h and introduce “oversteer”, where the driver swings left at the beginning of the turn by 1m and sweeps wider on the outside of the turn by the maximum possible (about 1.2m).

The corner now works with a single 12.5m curb radius, and the affected area is now just 25 square metres, with the crossing being extended just 2.5 additional metres. This variation does require a modification to the centre median however, which would require further investigation, but it could be quite feasible to shift the entire median about 6 metres to the east to make this movement possible.

Wide Right Turn

But wait, there’s more! It’s not uncommon in urban environments for trucks to start their turn one lane over from the right-most lane. In urban environments where truck turns are less frequent, it could be reasonable to expect this behaviour from truck drivers. In fact, this is actually a sign you’ll see on the backs of many trucks, like this one from SmartSign.com.

If you’re still not convinced this is a good idea, here’s a video of a truck doing it at an intersection in Ottawa (the corner radius is 10m).

Let’s see how a wide right turn affects the simulation.

With this final adjustment, the affected area is reduced to 6m2, less than 5% of where we started. The corner radius is 8.3 metres, and the increased pedestrian crossing distance is less than 1 metre. Similar to the last example, the oversteer movement requires some modification to the centre median, but to a much lesser extent.

Summary

Here’s the results of the four scenarios, shown side-by-side:

ScenarioAffected Area*Required RadiusIncreased Pedestrian Crossing Distance*
Smooth turn from curb lane to curb lane at 20 km/h133m215m + widen receiving lanes8m
Smooth turn from curb lane to inside lane at 20 km/h49m215m + widen receiving lanes4m
Oversteer from curb lane to inside lane at 5km/h with 1.0m entry offset and maximum exit offset25m212.5m2.5m
Oversteer from inside lane to inside lane at 5km/h with 0m entry offset and maximum exit offset6m28.3m<1m
*Compared to base case of 5m radius

The purpose of this demonstration is not to say that all corners should be small, but rather that, in the right context and with the right parameters, it is possible to design corners to be much more compact while still accommodating large trucks. Would you want to build an 8.3m radius at a rural highway off-ramp, or in a heavy industrial area? Likely not. But in any environment where a high volume of pedestrians are expected (or desired), it is essential that we consider methods like these to improve the walkability and safety of the street, and to preserve vital space for streetscaping and landscaping.

Disclaimers and Final Thoughts

  • I am a new user to this software and am still learning how to use it properly. It is possible that I made a mistake in this analysis. None of the findings of this post should be taken as fact or as professional advice!
  • There is also a risk to under-designing corner radii: trucks could sweep across the sidewalk, presenting a serious danger to pedestrians. For this reason, it’s likely best to establish a policy to create consistency in an area. For example, the City of Toronto has a city-wide policy for determining corner radii that likely helps contribute to a more predictable environment for truck drivers.
  • There is another solution may be more prudent in some contexts: prohibit large trucks altogether. When a designer does not need to accommodate a large transport truck, the corner can be much smaller. The City of Vancouver specifies a limited number of its streets as truck routes, which allows them to focus on creating better conditions for truck drivers on those routes, while using more compact designs elsewhere.
  • These techniques can and should be considered in other contexts as well, like the intersection of two local streets, or at driveway entrances. Assuming that the largest expected vehicle needs to nearly turn from lane to lane will lead to over-design in many cases.
  • In many cases, designers are restricted by the parameters set by the local municipality, and all municipalities take a different approach. For this reason, it’s important to push for strong pedestrian-friendly corner design policies on the local level, since this cannot be overcome with any single project.
Tire tread marks over the pedestrian area of a corner: evidence of a corner that is undersized.

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2 Comments

  1. Very interesting and relevant analysis, Matt! In the last option proposed, I’m wondering if this could actually be considered in the road design (use of second right-most lane for trucks to start the right turn). Considering that it’s likely to have other vehicles in the curb lane, particularly in a red light, could be there a way to make this work, maybe with the use of signalization, thinking of road safety as well?

    Like

    1. Very good question, and I think the answer is that it’s all about context. In a slow-moving downtown street, I would think a truck driver would slowly approach from the through lane with their indicator on, and very slowly initiate the turn once they see the right-turning vehicles beside them stopped. In a faster suburban street, this maneuver could be less safe, and a larger corner radius may be necessary. What I think this post demonstrates though, is that there is no “perfect” solution in any case!

      Like

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