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Road congestion implications of bigger HGVs
Prepared by MTRU for Freight on Rail

How much road space do heavy articulated lorries occupy?

This simple question is one which has to be answered to assess the impact of HGVs on congestion.  However, it is rarely calculated with sufficient accuracy or detail to be useful either for assessing the congestion impact of HGVs generally or the impact of specific changes in HGV length.

The standard way of approaching the issue is to compare the road space needed to that needed for a car.  This unit is called the Passenger Car Unit (pcu).  Of course, when traffic is so low that the HGV does not have any impact on the speed or spacing of other vehicles (for example in the middle of the night), it will not be causing congestion and the number of pcus is more or less irrelevant.

When there is an impact on other traffic, it is clear that this has two elements:

  • the actual length of traffic lane occupied +
  • the time that the lane is occupied.

If cars and HGVs always travelled at the same speed, the latter would not be an issue.  However, HGVs have lower maximum speed limits than cars and take longer to accelerate.  Thus in most traffic situations there will be a time adjustment, the most obvious exception being highly congested conditions with very slow moving vehicles.  In these circumstances all vehicles will be very close together and the lane length occupied will be very close to the length of the vehicle.

Lane length occupied in general traffic

In more normal conditions the lane length occupied by an HGV will have two main elements.  The first is the obvious one of vehicle length.  However, this is only the key factor in the case of a stop start traffic queue, where there is very little space between vehicles.  As an example, bumper to bumper a 16.5 metre long vehicle would take up just under 4 times the length of an average car (4.2 metres).

However, there is another element which is the additional space left by car drivers between themselves and an HGV (rear headway), and the space left by the HGV between itself and the vehicle in front (front headway).

Headways tend to be higher than a car length and are related to speed.  This is because the headways should be longer at higher speeds, to reflect longer braking distances.  In fact, HGVs have longer braking distances than cars and thus should leave a longer front headway 1.  Each vehicle’s road space requirement should include the front headway.

For example, at maximum speed and a front headway of 16 metres, the total length would be 32.5 metres for the HGV.  If the car used a front headway of 8 metres, this would make total of 12.2 metres for the car.  The pcu value would therefore be 2.7.  Allowing for the fact that the HGV speed is slower, the pcu value would be 3.2.  This is around the value often used for HGVs.

Effect on complex road networks

This simplified calculation may be useful for a single lane road with no junctions.  However, when junction capacity and multi-lane highways, which introduce overtaking, are included, the pcu factor becomes even more variable.

Considering junctions, issues arise such as right turning HGVs needing a far larger gap in opposing traffic and this can cause pcu equivalents in the high teens.  In local networks, difficult turns in either direction can require a very slow approach or some stop start manoeuvring.  Use of box junctions also means that long vehicles have to wait for a far larger gap straight ahead.

This means that local networks which are not designed for larger HGVs (existing or planned) would see very much higher congestion costs from HGVs by virtue of their size and length.  These would exceed the standard pcu value of around 3 many times over.

Impact of longer HGVs on high capacity road networks

In relation to changes in vehicle length, it is possible to create a range of pcu values for roads which are reasonably suited to HGVs, such as motorways.  In stop start conditions the change in pcu value would be close to the percentage change in length.  In situations where headways are a relevant factor, the change in pcu value would be close to the percentage change in length plus headway.  This would, however, mean that the acceleration and braking performance of the two different length vehicles would have to be the same.

Returning to the current proposals for changing the length of articulated HGV combinations, the following table has been prepared using the assumptions set out above.

Table 1
Increase in pcu value of longer HGVs in different road conditions

  18.75 19.4m 25.25m
Multi-lane grade separated dual carriageway, very low traffic 0 0 0
Free flow traffic, no junctions, but headways needed +6.9% +8.9% +26.9%
Stop start traffic, length only counted +13.6% +17.6% +53.0%

It has to be noted that no allowance has been made in the above table for the extra time and space required by the heaviest vehicles at junctions or on access roads to the high capacity networks.  This is because the values would be very variable according to road geometry, traffic conditions and vehicle acceleration and manoeuvrability.  However, they would be significantly higher.  This justifies separate consideration in terms of whether current lengths are suitable on many local road networks. 

The level of access to the road network permitted for larger types of HGVs has been the subject of discussion in other MTRU reports.  The overall conclusion was that the situation, even in terms of the current size and weight of HGVs, is far from satisfactory in terms of congestion, as well as other social and environmental impacts.  Clearly further work is needed to identify the impact on local roads, particularly junctions, which would enable a true picture of the congestion costs to be created.  It would also help to guide the level of access which should be permitted to the largest vehicles.

Implications for current size and weight proposals

It is clear that adjusting for these factors should be included in any traffic analysis of the impact of longer HGVs on national or EU road networks.

To illustrate this, the elasticity based modelling undertaken on Larger, Heavier Vehicles (LHVs) for the EU Commission by TML Leuven 2 can be used to calculate the impact on traffic.

In the original report, there were small decreases in traffic predicted as the result of introducing 25.25 metre LHVs.  This used an elasticity based approach to forecast any switch from rail and water to road, or any other traffic generation effects.  It also used assumptions about the increased carrying capacity of the LHVs.  The results were expressed as vehicle kilometres.  The results are shown in Figure 1 below.

However, adjusting for higher pcu values, the traffic impact would in fact be detrimental.  The figures for pcu kilometres have been used, instead of vehicle kilometres, to estimate a more realistic change in roadspace requirements and thus impact on general traffic.

A 50/50 split between free flow and congested conditions as set out in Table 1 would produce an average 40% increase in roadspace (lane length) needed by the new 25.25m vehicles.  This can be applied to the LHVs in the traffic flow at different levels of market share.  This is shown in Figure 2, which follows on from Figure 1.

Figure 1: Reductions in vehicle kilometres (no pcu adjustment) from introducing LHVs


Source: TML report, MTRU calculations

 

Figure 2: Reductions in vehicle kilometres from introducing LHVs,
adjusted for pcu roadspace requirements


Source: TML report, MTRU calculations

The effect of lesser increases in length would have a lower impact both on saving vehicle kilometres and on pcu values.  These would tend to cancel each other out and lead to the conclusion that, if vehicles were permitted to be longer, congestion is more likely to increase than decrease.

This method can be adjusted to allow for different combinations of pcu values but LHVs only have to need slightly more roadspace than existing vehicles to remove any benefits.  The example given shows that, using the TML elasticity model, it is extremely likely that the introduction of LHVs would increase roadspace requirements across the EU road network.

Overall conclusion

It is clear that, even without allowing for complex junction movements and other local network effects, adjusting for pcu value between different lengths of HGV gives a significantly different impact on roadspace requirements.  Studies of the impact of longer HGVs will need to reflect such an adjustment in any estimate of congestion effects.

MTRU November 2011
 

1. http://www.rospa.com/roadsafety/adviceandinformation/driving/hgv-truck-braking-systems.aspx

2. Effects of adapting the rules on weights and dimensions of heavy commercial vehicles as established within Directive 96/53/EC, November 2008, amended 2009

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