Bus Lanes

and why they don't cause congestion

Introduction

This chapter deals with bus lanes. It is a wide-spread belief that bus lanes cause congestion. "After all", goes the logic, "if you remove a lane from the road, you must reduce the capacity of the road, and hence the level of congestion goes up". There are two pieces of evidence to support this -

when bus lanes are put in, the traffic queue goes further back, and;
when the M4 bus lane was put in, there was massive congestion on the motorway.

Congestion

Where does congestion form? The best way to find out to watch the roads, and see where cars are queuing. In towns and cities, this almost always occurs around the junctions. Because the first vehicle takes some time to cross the stop line, other cars queue behind. On motorways, the congestion forms when the number of cars flowing along the motorway exceeds the capacity of the road. Almost all of the bus lanes are in towns and cities, and so we need to take a close look at urban junctions and how they work.

Single approach lane

Figure 1 shows a typical signalised junction, with left-hand drive, as in the UK. The signal head contains three lights, red amber and green. They go throught a sequence like this: green, amber, red, red/amber, green. The whole sequence is called a 'cycle', and the time it take to go through the sequence is called the 'cycle time'. The length of time that the green light is lit is called the 'green time'. The cycle time is the same for all approaches to the junction, but the green time is specific to each approach. Figure 2 shows how the cycle works.

When the signal head shows a green light (in fact, after a short delay), then the traffic moves through the junction. When the signal head shows an amber light (in fact after another short delay) the traffic stops moving. At most, 1800 cars can travel past the stop line in an hour (one every two seconds), although many fewer than that can travel through the stop line, because the signal head shows red or amber for some of the time, and so for some of the time the traffic will have stopped moving. Figure 3 shows the flow pattern across the cycle. Time increases from left to right, and it should be compared to Figure 2. As the traffic lights go green (Figure 2), the red line showing the flow goes from zero to the maximum, and it stays like that until the traffic lights go amber (Figure 2), and then the flow falls back to zero.

Dual approach lane

The original junction has now been upgraded to two approach lanes, as shown in Figure 4. This doubles the number of vehicles which can be move through the stop line. This is simply because the two lanes can be used at the same time. Figure 5 shows the flow pattern across the cycle.

Flared junction

The original junction could have been upgraded to a flared junction, as shown in Figure 6. This looks like it has two lanes, but in fact one of them is quite short. When the signal head shows a green light, the vehicles will move off from both lanes, although very soon all of the cars in the shorter lane will have gone through the junction. When the shorter lane has been emptied, the flow of traffic will slow to that for only one lane. How soon this happens depends on the length of the flare. If the flare is long enough, it will happen so late that the signal head will already show orange or red, and then the flared junction will behave like two whole lanes. If every cycle has 20 seconds of green time, at one vehicle every two seconds, this would be 10 cars, or 40m in length. If the flare was only 20m long, the flare would empty half way during the green time. This is shown in Figure 7.

A typical bus lane is shown in Figure 8. The buses can move down the bus lane without being impeded by other vehicles. However, the bus lane stops short of the junction, thereby creaing a flared junction. As long as the flare is long enough, in other words as long as the bus lane stops sufficiently far back from the junction, the bus lane will not interfere with the junction. This distance, called the set-back length, needs to be 2 metres for every second of green time. If this condition is reached, then the bus lane will not reduce the capacity of the road, nor will it increase congestion. A bus lane is just a smart way of improving bus journeys at no cost to car drivers, indeed, encouraging car drivers to leave the car at home and take the bus improves the congestion for everyone.

Queues

When a bus lane is put in, the queue length is observed to increase. This is simply because one of the lanes which the queue was spread across has now been removed. As long as the queue doesn't stretch back across a previous junction, there will be no ill effects due to the implementation of the bus lane.

Parking and Loading

Two considerations are kerb-side car parking and kerb-side loading for shops. If the near-side lane is used for parking, replacing the parking with a bus lane would be possible, provided that the parking is provided elsewhere. If the near-side lane provides loading space, this would also have to be provided elsewhere if a bus lane is built.

Merging Lanes of Traffic

At the start of the bus lane, one lane of traffic has to merge into the other lanes. If not enough distance is provided for this process then congestion will be formed at this point, rather defeating the point of having a bus lane. DfT (Department for Transport) guidance is a taper of no sharper a gradient than 1 in 10. This means that a 3m bus lanes requires a taper no shorter than 30m. This arrangement is shown in Figure 9. Continuation of a bus lane after a side road can then be provided using a curved line to indicate that the bus lane starts again at this point.

Figure 9: Permitted bus layout (source: DfT)

Bus Lane Location and Bus Gates

Bus lanes are usually located in the kerb-side lane, which is where bus stops are usually provided. However, it can make sense to position the bus lane on the offside, for example if the bus is shortly thereafter to turn right (with the UK's left-hand drive system). The problem with positioning the bus lane on the offside position is that it means that passengers find it much harder to get access to the bus stop. They either have to cross what is likely a very busy road, or else footbridges or subways have to be provided.

An alternative approach is the bus gate. Bus gates come in various types, some allowing cars, and some not. In this particular case, the bus gate consist of a set of traffic lights some way short of the final stop line. Normally the traffic lights of the bus gate are green, and the traffic flows through the bus gate to the stop line of the main signalised junction. This is shown in Figure 10a.

When the bus passes over a detector in the bus lane the traffic lights of the bus gate are set to red, so that the queue of cars is held back, and the bus can easily move to the desired lane during the next cycle of the traffic lights at the main junction. This is shown in Figure 10b.

This is at no cost to the car drivers, who are merely queuing in a different place. Usually the bus gate is set back from the main junction at a distance set so that all of the traffic in front of it will clear during one green period of the traffic lights at the main junction. The bus then joins the back of the waiting queue.

The M4 bus lane

The M4, as it was pre-bus lane, consisted of two widths of road, a section of 6-lane motorway, and a section of 4-lane motorway. When the three lanes of traffic were restricted into two lanes, with this motorway design, a phenomenon called a 'shock wave' was formed, which caused massive congestion. The plan was then to decongest the motorway by removing one lane of traffic and making the motorway 4-lane throughout - thereby creating a bus lane. Shortly after the bus lane was installed, there was a serious crash which closed the motorway, although most people put the congestion down to the bus lane. Why didn't the government of the day defend themselves, and explain how the bus lane worked? I don't know.