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Conventional traffic management
A first principles assessment Why introduce Traffic Management? Conventional Traffic Management schemes include one-way streets, junction redesign, banned turns and controls of on-street parking (Thomson, 1968; May 1997; Fitzpatrick et al 2005). The primary objectives of Traffic Management are to increase traffic capacity and safety. Other potential objectives include environmental protection and reallocation of road space to improve public transport and pedestrian movement. Route Restrictions Route Restrictions are measures that change the direction and movement of traffic. The most common example of route restrictions pertaining to general traffic is the conversion of a two way street system into a one way system. Even so, there is an ongoing debate regarding whether one way streets or two way street systems are better. Supporters of one way systems (e.g. Stemley, 1998; Cunneen and O’Toole, 2005) point out that one way street systems are safer primarily because the conflict points are reduced in one way systems. In addition they argue that capacity is increased due to one way systems. At the same time, the literature (Walter et al, 2000 ; Lum and Soe, 2004) also points out that one way street systems impose additional turning movements at junctions and increases in vehicle miles travelled, both of which can increase the chance of accidents. In practice, some city centres have taken the initiative to convert one way systems into two way systems (especially in North America e.g. Hart 1998 and examples pointed out in Walker et al, 2000). A comparison of one-way versus two-way street systems for city centres was evaluated and an evaluation methodology for considering two-way street conversion was presented in Walker et al. (2000). Right of Way Restrictions These generally prevent or regulate certain movement of traffic in junctions. In this category are measures that range from banning right turns, through priority control and signal control to complete junction re-design. Junction redesign itself can range from painting simple unmarked junctions with priority markings to conversion of priority (i.e. give way) junctions into signalised roundabouts or gyratory systems. In some cases, junction redesign might arise as part of a new road scheme or new development proposal. This is generally the case when anticipated post-scheme traffic flows might be much higher. For example at high traffic volumes, roundabouts do not operate well (Wright and Ashford, 1998). However, the most often cited reason for junction redesign is safety partly because accidents tend to occur primarily at junctions1. With low volumes, roundabouts might be particularly useful since research has found that conversion of priority junctions to roundabouts can reduce accidents. As the volume of traffic increases, signalised intersections become safer than priority junctions. With priority control, traffic on the minor road needs to “give way” to flows on the major road2. Drivers on the minor road therefore have to wait for a gap in the traffic and the probability of adopting risky driving behaviour tends to increase with traffic flows. Hence it is usually the case that as traffic flow increases, priority junctions are converted into signalised intersections. In summary therefore the objective for junction redesign is generally to support anticipated increases in traffic demand at intersections and to improve the safety record at junctions. Principles and guidance on junction design can be found for example in IHT (1997); Wright and Ashford (1998); Slinn et al (2005). Parking Restrictions Parking restrictions (encompassing waiting and stopping restrictions) allow for clear operation of a carriageway. If vehicles are parked on street, the effective throughput or capacity is reduced, particularly where parking occurs close to junctions. In addition, drivers slow down naturally when passing parked vehicles due to the potential of opening doors etc. Parked vehicles can also block sightlines and hence obscure other vehicles or pedestrians, increasing the risk of accidents (Yousif and Purnawan, 2004). The same applies when vehicles are waiting (for any purpose including loading). In cities, where there is competition for street space, parking on the street is provided at the cost of a general decrease in mobility (Forbes, 1998). Parking leads to congestion as the driver slows down to search for a free space, engages in the act of parking itself, or searches for a gap to move off (Yousif and Purnawan 1999). Hence it has been a policy in many city centres since the 1950s not to allow on-street parking especially during peak hours (Forbes, 1998). Parking restrictions can also serve to speed up the journey time of buses and enable the travel time by public transport services to be competitive against that of the private car. While bus services may never approach the level of segregation of light rail systems for free running, traffic management measures such as clearways and bus lanes allow a degree of separation for buses and such traffic management concepts are always a feature of current interest in bus rapid transit. Such clearways occur at bus lanes where stopping/waiting/loading and unloading is prohibited). Enforcement can be by means of bus lane cameras or camera devices fitted on to buses. The availability of convenient parking is a major factor influencing the decision to drive to that destination (DfT, 1996). Thus restricting parking space can have an impact on modal shift, as discussed under parking controls.
Demand impacts
Short and long run demand responses If service levels are reduced then responses can be assumed to be diametrically opposite to those presented here.
Supply impacts The change in the amount of supply (effective capacity of the road network) depends on the measure applied. Route Restrictions: One way streets increase network capacity, provided that traffic is not diverted onto excessively long routes, by simplifying junction movements, making more effective use of available road width, and removing friction with opposing traffic. Typical schemes may increase capacity, in vehicle-km/h, by 10% to 15%. Right of Way Restrictions: Conversion of priority junctions into roundabouts or signalised junctions can increase capacity and reduce delay for minor movements. However, the increase in capacity is critically dependent on the design of the modified junction. Increases in junction capacity, in vehicles per hour, of 10% to 20% are possible. Parking Restrictions: Implementing (and enforcing) parking restrictions will increase capacity for moving traffic, particularly where parking is removed on the approaches to junctions. In such cases, increases in capacity, in vehicles per hour, of up to 40% are possible. Financing requirementsThe financing requirements depend on the particular measure applied as well as the coverage of the scheme (effective route length or number of junctions). Costs will be much higher if additional land is needed, but this would make it no longer strictly a traffic management scheme. Route restrictions typically require only moderate costs for implementation, largely involving signs and markings, and enforcement costs are typically low. Right of way restrictions are typically more expensive to implement since they will require some carriageway reconstruction (for roundabouts) or equipment (for signals); again, they are inexpensive to operate. Parking restrictions cost little to implement, but have a continuing enforcement cost if they are to be effective. Expected impact on key policy objectives It is clear that the actual impact of conventional traffic management depends on the measure used. Equally, it is crucial to emphasise that whilst a measure may solve a problem in a local area, it may lead to the emergence of problems elsewhere. This is the same view articulated in DfT (1996) where it is stated that “Traffic management schemes can affect vehicle emissions by altering the volume, speed and composition of the traffic stream and the driving pattern (steady speed, stop/start, acceleration and deceleration). There is also the need to recognise that, whilst traffic management schemes may be able to reduce the impact of traffic on air quality in the immediate locality, they may have a relatively small city wide effect.”
Expected impact on problems
Expected winners and losers
Barriers to implementation One of the primary difficulties with traffic management implemented on its own rather than in a package of measures is that traffic management might introduce some adverse impacts such as traffic rerouting and environmental intrusion onto quiet streets. Even if journey speeds are increased by traffic management measures, the journey length may have increased and this could result in no change in travel times3. In addition, there could be reduction in accessibility for certain users e.g. when one way streets are introduced, buses might have to be rerouted. Emergency services might also adversely affected (Stemley, 1998). It must be borne in mind that while the local (i.e. treated) area generally improves, there are often adverse impacts of these measures on other areas (e.g. parallel routes). Similarly whilst vehicles are not generally obliged to stop at “priority junctions” when conflicting traffic is clear or they have priority (and hence avoid idle queuing), vehicles have to stop at the red light and subsequently accelerate, resulting in excess fuel consumption that produces additional carbon emissions (Peters et al, 2009). Parking restrictions could potentially increase the search costs for parking as spaces (which could be in front of shops etc) may be removed. Vehicles may then have to intrude on neighbouring streets (if the ban is not applied there). It must also be recognised that traffic management effectively increases the supply of road space to some road users (although it might reduce it for others). To the extent that the supply of capacity for private car users is increased then this implies that the cost of driving is actually reduced. By encouraging a smoother flow of traffic, there is potential to therefore increase the total number of trips made by private car. Obviously this will depend on the exact elements of the package implemented.
Text edited at the Institute for Transport Studies,
University of Leeds, Leeds LS2 9JT
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