Tuesday, December 10, 2019

The prospect of limiting carbon emissions

I recently went to a meeting hosted by Carbon Neutral Cambridge on developing a Local Transport Plan that would get us towards being carbon neutral.

Quick summary: the measures proposed by local authorities (specifically Cambridge, but I suspect it's not that different elsewhere) won't get us even close to being carbon neutral by 2050, and likely not ever.

Given that, realistically, any hope we have of preventing the worst consequences of climate change means that we need to be carbon neutral by 2030, this is somewhat disappointing.

There was then an analysis of a fairly radical scenario, and even that struggled to just about reach carbon neutrality by 2050.

And, what's worse, both scenarios were pretty optimistic in terms of timescale and takeup of technology. The likelihood is that there will be dithering and delays, pushing it back even further.

(And there's the incorrect assumption that switching to electric vehicles will reduce carbon emissions. On its own, it won't, it requires the world to move far faster to renewables or nuclear than we are right now.)

The other thing that isn't immediately obvious is that, right now, the carbon footprint of moving people about is the same as that for moving goods about. You have to solve both.

We then broke up into focus groups. I hate this (it's just me) - the chances are that either nobody will have anything to say so you spend the time twiddling your thumbs, or there's so much you don't even scratch the surface. On this occasion, we didn't even really get started. It's a huge topic.

But our group (although we did have Peter Dawe sidetrack us with his CitiPod) thought about a couple of priority areas:
  • For people: reduce the need to travel, and the use of remote working
  • For goods: move long distance traffic to rail, and have local distribution hubs
What is clear, though, is that we need significant modal shift from current transport systems to more energy efficient ones. Yes, this means getting rid of cars in their current form. Difficult, given the rather dire state of public transport and the lack of investment in it (although they can always find a few billion to build roads to induce extra demand and cause more congestion and pollution), but necessary.

Friday, December 06, 2019

The effect of passenger boarding on bus services

We all know the saying "you wait ages for a bus, and them 3 arrive at once". But why is this?

The basic reason is simple: once a bus starts to run late, there are likely to be more passengers waiting at stops, slowing it down. Because it's running late, the bus behind it has fewer passengers waiting when it gets to stops, and catches it up. There's a strong feedback loop that drives well-separated buses further apart, and closely separated buses get pushed together.

As a passenger, I noted the characteristics of a number of journeys I took over the summer. These were during the day, so avoid the rush hour.

Very roughly, what I see is that on a 24 minute journey between home and the centre of Cambridge, we spend typically:

  • 12 min actually moving
  • 6 minutes boarding
  • 5 minutes stopped due to traffic/lights
  • 1 minute unboarding

This doesn't account for the unboarding at my destination stop (terminus).

The boarding is much higher than unboarding for the simple reason that you just get off. When you board, you have to pay, show your ticket, or may have queries.

What was also true was that the distribution of boarding times isn't simple. The majority of passengers board quickly in the 10-20s range (there are two peaks, those who've prepaid and are just showing their card or ticket, and a somewhat slower group who have to pay). However, there's a long tail: a small number of passengers have much longer than average boarding times. I've seen some take several minutes - maybe they don't know where they're going, they don't have the right change, they don't understand the system.

The traffic lights are also rather variable. If you get caught by the lights, you can get a wait of several minutes. (The junction at the Catholic church in Cambridge in particular can cause large delays.)

Unlike a train, which pulls into a station, opens its doors for a fixed time, and then goes, a bus stops as long as necessary to let its passengers on or off. This, coupled with the traffic delays, means that wait times are highly variable.

This also means that the predicted arrival times as shown on bus stops by the real time traffic displays simply can't be terribly accurate.

I knocked up a quick model of my bus journey with the observed distribution of boarding and wait times, and (as expected) it comes back with results that aren't dissimilar to the characteristics of actual journeys:
  • Just allowing for the fact that the number of passengers on the journey is random gives a variance of +/- 3 minutes
  • Allowing for the feedback of delays early in the journey causing longer queues later gives a variance of +/- 4 minutes
  • Allowing for the previous bus as well gives a variance of +/- 5 minutes
Given this variability, it's hardly surprising that adherence to the bus timetable is notional at best.

One way to deal with this is to add waits to bring the bus back in line with the timetable. (You have to add waits, you can't remove time unless you have a time machine.) And my 24-minute journey is allowed 30 minutes, so we routinely stop 2 or 3 places along the route. But having to build in this extra wait time is pure waste.

What you can also see is that if the bus were to be full (in other words, taking on 60 people) then it's not entirely unreasonable to require 15 minutes for everyone to board. Those who have stood in a queue in the city centre on a busy evening or a Saturday afternoon will have seen the buses stationary at the stop for this sort of time. Even with the slop in the schedule, it's almost impossible to keep to the timetable if the bus is full. (Especially as those tend to be times when the roads are more congested.)

What if you could reduce boarding time? This gives you a double win: less boarding time makes the journey quicker, but also gives less variability, so you need to build in less slop. It's better for passengers, who get quicker and more reliable journeys, and it's better for the bus operators who make much better utilization of their buses and drivers.

Looking at my journey just to be specific, optimization of loading could almost halve journey times and double efficiency. At busy times on short routes having a second staff member check tickets - rather than forcing the driver to do so - is obviously a win. (This doesn't have to be a conductor, who would check or sell tickets while the bus is moving after everyone has boarded. It could be an inspector at certain stops who validates tickets of those in the queue. When I was living in Toronto some of the busier bus stops were closed interchanges, you paid as you entered the system and didn't need to be checked at the point of boarding at all.)

Eliminating charges entirely would have a similar effect. People just walk on without having to stop. Again, the system is far more efficient as a result.

One of the other minor issues with buses in Cambridge is only having one door. Those wishing to board have to wait for everyone who wants to get off to do so; many other systems have separate doors allowing boarding and unboarding to operate in parallel.

The actual process of issuing tickets has improved, I think. Smart cards and contactless payments are much quicker than the old cash and paper tickets.

But what's interesting here is that improving the efficiency with which passengers board a bus does have the potential to significantly improve journey times, reliability, and the efficiency of the bus system.