ROAD GEOMETRIC DESIGN MANUAL ROAD GEOMETRIC DES...

The adopted criteria describe design values for through travel lanes, auxiliary lanes, ramps, and turning roadways. There are also recommended widths for special-purpose lanes such as continuous two-way left-turn lanes. AASHTO also provides guidance for widening lanes through horizontal curves to provide for the off-tracking requirements of large trucks. Lane width does not include shoulders, curbs, and on-street parking areas. Table 3 summarizes the range of lane widths for travel lanes and ramps.

In a reduced-speed urban environment, the effects of reduced lane width are different. On such facilities, the risk of lane-departure crashes is less. The design objective is often how to best distribute limited cross-sectional width to maximize safety for a wide variety of roadway users. Narrower lane widths may be chosen to manage or reduce speed and shorten crossing distances for pedestrians. Lane widths may be adjusted to incorporate other cross-sectional elements, such as medians for access control, bike lanes, on-street parking, transit stops, and landscaping. The adopted ranges for lane width in the urban, low-speed environment normally provide adequate flexibility to achieve a desirable urban cross section without a design exception.

Designers should understand the interrelationships among lane width and other design elements. On high-speed roadways with narrow lanes that also have narrow shoulders, the risk of severe lane-departure crashes increases. Drivers on rural two-lane highways may shift even closer to the centerline as they become less comfortable next to a narrow shoulder. At other times, they may shift closer to the shoulder edge and are at greater risk of driving off the paved portion of the roadway (and over potential edge drop-offs) as they meet oncoming traffic.

Horizontal alignment is another factor that can influence the safety of lane width reductions. Curvilinear horizontal alignments increase the risk of lane departure crashes in general, and when combined with narrow lane widths, the risk will further increase for most high-speed roadways. In addition, trucks and other large vehicles can affect safety and operations by off-tracking into adjacent lanes or the shoulder. This affects the safety of other drivers, as well as non-motorized users such as bicyclists who may be using the adjacent lane or shoulder. It is important to understand this interaction of design elements when a design exception for lane with is being evaluated.

Lane width has an effect on traffic operations and highway capacity, particularly for high-speed roadways. The interaction of lane width with other geometric elements, primarily shoulder width, also affects operations.

Smart roads, AV and CAV are emerging technologies that represent the new paradigm of mobility. To support the public and private road operators better prepare themselves to implement these technologies in their respective existing or planned infrastructures, there is an urgent need to develop an integrated analysis framework to evaluate the impact of these novel systems on road capacity and safety in function of different market penetration levels of AVs and CAVs. The research focuses on novel smart road geometric design and review criteria based on the performance of AVs and CAVs. The case study of one of the first planned smart roads in Italy has been analysed.

In this research the design and review criteria (formalised in [4]) founded on the performance of emerging AV and CAV technologies are used for the design review of the existing Italian A19 motorway that will be one of the first smart roads in Italy. In addition, the expected increase in motorway capacity is estimated.

In this section are given some smart road design criteria founded on the potential performance of AVs and CAVs [8]. They were obtained by modifying those traditionally employed in highway engineering in function of the so-far known potential performances of AVs and CAVs.

Table 2 shows the summary of the smart road design criteria used in this research, including the design of straights, horizontal circular curves, transition curves (clothoids), crest vertical curves and sag vertical curves.

In conclusion, the study aims to cover the research gap in the field of smart roads geometric design and capacity estimation. It is worth underlining that the results of this study are affected to the hypotheses assumed in the proposed and adopted closed-form models in particular as concern the potential performances of AVs and CAVs. Therefore, the closed-form models from this research and the main results of the analyzed case study may represent only one of the first tools to include the emerging Smart road, AVs and CAVs technologies in the decision processes concerning the novel digitalized-road planning and designing.

More reliable mathematical models could be considered in future researches as well as the study of the effect of emerging technologies on other geometric design parameters of smart roads (e.g. the lane widths which may be smaller than those currently used) and the sustainability of such innovative transportation systems.

There may also be cross-linking to standards from other countries, such as in the graphic at the beginning of this post (related to data on side friction), where it may not be clear where the orignal data came from. In general, when a standard quotes values for side friction you cannot be sure that the values represent similar regional or economic conditions, are based on research values from the same century, relate to the same parameters or conditions (operating speed or design speed dry or wet road surfaces) or are based on values which have not been modified to include e.g. factors of safety.

Adding to the problem of many standards based on different road types etc there is also the problem that often different administrative levels in any one country publish their own design standards. For example:

In the last two blog posts I suggested that any set of suggested design values should be seen in context, and not just taken as abstract values. For example, a value for minimum horizontal radius of 437 metres at 100 km/hr may have been prepared for USA car drivers on well-maintained roads. Now take a look at the following table, which gives some values for minimum horizontal radius taken from design guidelines published in the USA and in a number of other countries. The USA values and the values from the other countries shown seem very similar.

Horizontal radius is one of the basic parameters used in the geometric design of roads. Most standards and guidelines include tables of values and notes on the topic. One such standard is the Austroads document Guide to Road Design Part 3: Geometric Design. IMO the Austroad documents are among the best presently available. 59ce067264

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