One of the more common arguments we’ve seen against having light rail to the airport, even though it’s not only about the airport, is that heavy rail is faster. A simplified version of the argument is that our heavy rail trains can travel at up to 110km/h so that is better.
This post isn’t actually about the airport, or even heavy vs light rail, but instead about impact that faster speeds have on travel times. It comes as the by-product of another post I’m working on, which I hope to publish soon. I’ve created a model to estimate how long it will take to travel between stations based on the distance and the maximum speed allowed. While working on it, I started to notice some factors making a bigger difference than others. So, I decided to look at that in a bit more detail which lead to this post.
The model is based on some information AT gave me about our electric trains at the time they were first being built. It was reported at the time that the trains are capable acceleration of up to 1m/s², however this information shows they can only sustain that up to 35km/h, after which the rate drops off. The same thing with braking too, with the trains able to brake at 1m/s² up to 50km/h. The profile for both these is below, although it also assumes flat tracks and normal conditions.
So how does that translate into travel times? For the purpose of this post, I’ve assumed that regardless of the type of rail, there’d be a broadly similar profile to above. I’ve then compared travel at various maximum speeds over two distances, 800m and 2km. I chose 800m as it is similar to what the average spacing of the proposed light rail stations on Dominion Rd will be, while 2km is closer to the average between Dominion Rd and the airport as well as on most of our heavy rail network. I found the results fascinating.
For this I compared maximum speeds between 30 and 80km/h. The travel times varied between 1:01 and 1:44 however, as you can see the gaps between them aren’t consistent. As a result, a vehicle travelling at a maximum speed of 70km/h would arrive just one second after one travelling at up to 80km/h, followed another three seconds later by the one at 60km/h.
In case people don’t quite believe there is only one second difference between 70 and 80km/h, think of it this way. Something travelling at 80km/h will cover 22.2m every second while one at 70km/h will cover 19.4m every second. In the 14 seconds it takes to accelerate from 70km/h up to 80km/h and then brake back down to 70 again there is a difference in the distance covered of just 25m. So it takes the slower vehicle just over a second longer to make that up.
As the speed increases you can really see the impact of those slower acceleration times with it taking longer to get from 100 to 110km/h than it does to get from 0 to 40km/h. I was surprised when this came out again at almost exactly the same as the graph above, only on a slightly bigger scale. In this case, a maximum of 90km/h seems to be the sweet spot with a vehicle arriving just four seconds behind one capable of reaching 110km/h.
So why is all of this important. Perhaps unsurprisingly, what these two examples show is that vehicle speeds need to be considered in relation to the network they’re operating on. For most of our rail network, light or heavy, existing or proposed, there is actually not a huge benefit from being able to travel much faster than about 90km/h. On urban networks the rate of acceleration is actually more important than the maximum speed. A high top speed only really becomes important for longer distance journeys where there is the ability to cruise at that speed for an extended period of time.
I look forward to sharing the rest of the outcomes from this model soon.