Signalised Junctions

an analysis of the advantages and disadvantages of signalising a junction

Introduction

This web page was written in response to an opinion on some web-sites which goes like this - "I was driving in my car the other day, and I noticed how all these signal junctions were slowing me down. I think it's deliberate - they're causing congestion so they can congestion charge us". Congestion charging is dealt with elsewhere on this web-site. This web page looks at the effect of signalising a junction, how much it costs, and whether signalising a junction creates advantages or disadvantages to road users.

The Experiment

The experiment chosen concerns a T-junction, as shown in Figure 1 and Figure 2. It comes in two flavours - 'priority', with the traffic on the minor road giving way to traffic on the main road; and signalised, where all of the traffic must obey the traffic lights. Both kinds of junctions are left-hand-drive, although the same principles apply to right-hand-drive junctions. All of the lane widths are 3.5m. Good visibility along the roads is assumed.

In the priority form, there is no right turn lane on the main road, so that traffic turning right into the minor road blocks the traffic which wants to go straight ahead.

In the signalised form, first the two directions on the main road run together, with traffic turning right into the minor road having to give way to traffic coming the other way. Then the traffic on the minor road can move. In the USA this is called '2-phase' operation, and it is the most common way of signalising a T-junction in the UK. The traffic signals in the experiment are set up in such a way as to minimise the delay to all vehicles using the junction.

Multiple tests of each junction were done. Traffic flows were increased a small amount at a time. The size of the flows through the junction were determined by what was happening on the minor road. Equal flows of traffic followed these routes:

Then the flows along the main road were twice this in each direction.

For example, supposing that 100 vehicles went in each of the four directions listed above. Then 200 vehicles would travel in each direction along the main road. This would correspond to 100 on the bottom axis of the graphs below. The flows in this example are shown in Figure 3.

The priority junction was modelled using PICADY, a program from TRL. The signalised junction was modelled using LINSIG, a program from JCT.

Results

The results are shown in Figures 4 and 5. Figure 4 shows the delay suffered by all vehicles at the junctions, and Figure 5 shows the RFC value for the junctions. RFC is an abbreviation of 'Ratio of Flow to Capacity', and is calculated as the traffic demand divided by the junction capacity. When the RFC is 100%, it means that the junction is fully used - this is shown on the graph as a thick red line. Each arm of the junction will have its own RFC, and in this case the worst RFC for the junction was taken, as is customary.

At low traffic flows, the delay for the priority junction is lower than that for the signalised junction. This is because with the signalised junction, drivers end up waiting for a green light, whereas with the priority junction there is no such delay. At higher traffic levels, the priority junction is quickly overwhelmed - the RFC value quickly rises, leading to high delays per vehicle. Of course, this is an average delay. The westbound lane, as seen in Figure 1, will always flow freely, although the other lanes of traffic will experience high levels of delay.

By contrast, the signalised junction does much better, and once the levels of traffic start to build, the signalised junction carries twice the amount of traffic for a given level of delay. Also, the delay is evenly shared out amongst all of the drivers. The traffic signals are sharing out the junction amongst all of the cars, in the way that a prioirty junction simply cannot.

The price of signalising a prioirty junction like this is approximately £40,000, with additional on-going maintenance.

Summary

For light traffic flows, the priority junction is a better design having lower delays for car drivers. At higher traffic flows, signalisation of the junction is required, and leads to lower delays than the equivalent priority junction, despite what it feels like when driving a car.

Most estate road junctions are of the priority type, since the traffic flows are very light, and cannot justify the expenditure on traffic lights. Most major junctions are signalised in order to provide enough capacity and to reduce congestion.

It also follows that the continental system of switching off traffic lights when the traffic flows are light will lead to measurable reductions in delay for the vehicles using the road. This is particularly true of traffic lights on roundabouts, since a roundabout is no more than a ring of priority junctions.