1. Introducción
2.6 Juegos de Referencia
On most urban roads (including arterials and sub-arterials) vehicle speeds are regulated by factors other than the physical characteristics of the road. Other vehicles, traffic control devices at and between
intersections, and mid-block friction caused by vehicles manoeuvring between lanes all affect the operating speeds of urban roads. Whether the vehicle is travelling during peak, inter-peak or off-peak times also has a large impact on the operating speed due to issues such as congestion, which can change over time as the available roadway capacity is utilised by increasing traffic demand. Given these issues and those described in Commentary 3, no model to determine the operating speeds of vehicles in urban areas has been formally validated for use in Australia or New Zealand.
[see Commentary 3]
1 Department of Main Roads (2007) has been superseded and Figure 3.1 has not been carried forward into Queensland Department of Transport and Main Roads (2015d).
Where possible, designers should determine the operating speeds of urban arterial and sub-arterial roads through speed measurement of the road under consideration, or of a comparable road. Although it is acceptable for designers to estimate vehicle speeds in urban areas using the Operating Speed Model consideration needs to be given to the validity of the results, given the issues described in Commentary 3. However, the experience of some road agencies has shown that the Operating Speed Model provides comparable results to the operating speeds measured on arterial and sub-arterial roads under light traffic conditions. Other issues to be considered when determining the operating speeds of urban roads are:
the functional classification of the road
topography
land use and abutting development including the amount of direct access to the road
driver expectation of the speed limit.Road agencies may achieve a greater understanding of operating speeds on urban roads by undertaking a series of systematic speed surveys to establish the relationship between speed limit and operating speed, and changes in that relationship over time. Speed data may be available from road agencies or obtained from local traffic classifiers. For example, a review of speed surveys carried out over a number of years by VicRoads found that:
As the operating speed on the Melbourne metropolitan arterial network had effectively stabilised at very close to the posted speed limit, the adoption of an operating speed equivalent to the posted speed limit is acceptable for design purposes on metropolitan arterial roads.
An operating speed of 10 km/h greater than the posted speed limit should be adopted for metropolitanfreeways/motorways. When necessary, a risk-based approach may be adopted to assist in choosing an appropriate operating speed (less than 10 km/h greater than the posted speed limit) for a project where design controls warrant an alternative approach to ensure that a value-for-money design solution can be developed. Use of an Extended Design Domain (EDD) approval process is required.
It might be reasonable to adopt an operating speed equivalent to the posted speed limit for arterial roads in major regional cities, subject to validation.
An operating speed of 10 km/h greater than the posted speed limit should be adopted for rural highwaysand high speed rural roads together with a risk based approach and use of an EDD approval process when an alternative approach is proposed.
Designers should consult with the relevant road agency guidelines when determining the appropriate operating speeds for urban roads. In the absence of any other evidence, designers should adopt an
operating speed 10 km/h higher than the posted speed limit as this often reflects the desired speed of drivers and provides for a factor of safety given the limited information surrounding the estimation of urban operating speeds. Table 3.1 lists values of typical urban posted speed limits for a range of urban road types.
Table 3.1: Typical posted speed limits (km/h)
Road type Posted speed limit (km/h)
Freeway 100–110
Dual carriageway with service roads 80–90
Dual carriageway without service roads 60–80
Single carriageway two-way arterial 60–80
Collector roads 50–60
Residential streets 50
One-way service roads 50
Note: Some urban freeways may have a posted speed limit of 80 km/h, either by permanent signs or variable speed limit signs.
The following sections provide some considerations for urban road types and the expected driver operating speeds.
3.3.1 Freeways (Access Controlled Roads)
These are roads that are intended to provide a high quality of service for high traffic volumes, and need not be designated as freeways (some road agencies refer to these roads as motorways). They are characterised by having full control of access, median divided multi-lane carriageways, grade separations and
interchanges. Vertical alignments tend to have flatter grades in order to minimise the difference in speed between cars and trucks.
The standard of horizontal and vertical geometry permits (and indeed encourages) uniform speeds. On these types of roads, the desired speed will equal the operating speed and is typically close to 10 km/h above the posted speed limit. Given the uniform operating speed, a single design speed greater than or equal to the operating speed can be used.
Interchange ramps
Interchange ramps may be characterised by:
a low speed terminal with an intersecting road and a high speed terminal where vehicles enter/leave amajor road (access controlled road)
a connecting roadway that allows a vehicle to turn from one major road to another without stopping andincludes a high speed terminal at each end of the ramp.
For the purpose of determining ramp lengths and matching all geometric parameters to the operating speed at any point under a range of operating conditions, the design of these ramps requires speed profiles to be determined on:
the acceleration or deceleration of vehicles under free flowing conditions
ensuring that the end of queued vehicles (even back from any design queue length) is always visibleunder different deceleration conditions
allowing for acceleration profiles in times of ramp metering. Refer to the Guide to Road Design Part 4C:Interchanges (Austroads 2015e).
For ramps between two high speed terminals, the operating speed that can be expected on a given horizontal curve radius will typically be at or near the limiting curve speed (refer to Appendix C). This is a normal characteristic of these ramps. Loop ramps need to be designed in accordance with the Guide to
Road Design Part 4C: Interchanges (Austroads 2015e).
For connections where it is obvious to drivers that they are changing from one road to another, drivers expect to slow down more than they would generally be prepared to if they were approaching a curve on a ‘through alignment’.
Section 6.4.1 of Part 4C, i.e. (Austroads 2015e) establishes the design domain for the operating speed on the smallest or controlling curve on the ramp, based on operational experience. Invariably, the controlling curve will be smaller than drivers are familiar with on the major road, and drivers will not tend to drop below a speed that they are comfortable with for the curve radius. Therefore, the Operating Speed Model should be used to predict the operating speeds on connecting roadway ramps.
3.3.2 High Standard Urban Arterial and Sub-arterial Roads
These roads have a standard of horizontal geometry where all elements are suitable for the desired speed. The standard of geometry is intended to promote uniform operating speeds in the interests of operational efficiency. Actual vehicle speeds however, vary much of the time due to the interaction of other vehicles and the need to stop at traffic signals. Even so, drivers tend to revert to the desired speed when they get the chance and traffic signal phasing and coordination reduces the number of times that they need to stop. On these types of roads, the desired speed will equal the operating speed and is typically close to 10 km/h above the posted speed limit. A single design speed greater than or equal to the operating speed can be used.
3.3.3 Urban Roads with Varying Standard Horizontal Curvature
This category covers any urban road where it is not possible to have all horizontal curves of a size that is suitable for the desired speed. The posted speed limit is usually 80 km/h or less, and the desired speed of drivers is often about 10 km/h higher than the speed limit.
On these roads, the operating speed and hence design speed, needs to be determined for each element of the road, with the aid of the Operating Speed Model. It is necessary to ensure that an alignment has an acceptable level of geometric consistency by limiting how much drivers have to slow down for a curve.
3.3.4 Local Urban Roads
This category of road would usually be developed for low speed conditions, typically with a default urban speed limit of 50 km/h or less. In more recent developments, the horizontal geometry and various traffic management devices are typically used to constrain vehicle operating speeds. These roads are also characterised by large numbers of property accesses. As these roads are generally managed by municipal governments, designers should consult with the relevant authority when designing local roads.