Appunti Design of Transportation Infrastructures 2020

Norme per le piazzole di sosta

Lay-bys devono essere fornite ogni 1000 m su entrambi i lati delle strade di classe B, C, F-ext. Le piazzole di sosta sono consigliate anche per le strade di classe A-ext e A-urb, anche se è presente una corsia di emergenza.

Corsie di sorpasso

Lungo una salita ripida, deve essere fornita una corsia di sorpasso dedicata ai veicoli pesanti (e più lenti) quando:

• Si riduce il livello di servizio
• I veicoli pesanti riducono del 50% la velocità delle auto passeggeri, seguendo la tabella sottostante.

Progettazione dell'allineamento verticale

In caso di un nuovo allineamento su una strada o ferrovia esistente H = H + Hà fs strLM Ing. Civile PoliToM. Bassani: Progettazione delle infrastrutture di trasporto (a) Francesco Torre, 2020/2021 23

In caso di nuovi allineamenti sotto una strada esistente:

In caso di sovrapposizione a un fiume, è necessaria una distanza verticale sopra l'elevazione idrometrica massima del fiume (basata sul periodo di ritorno di progettazione).

INTERFERENCES ANNOTATIONS FOR RIVERS AND EXISTING INFRASTRUCTURES

RIVER

EXISTING RAILWAY

NEW OVERPASS

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M. Bassani: Design of Transportation Infrastructures (a)

Francesco Torre, 2020/2021 24

INTERSECTIONS

From AASHTO Green Book

Area where two or more highways join or cross, including the roadway and roadside facilities for traffic movements within the area. Each of these elements is an intersection leg. An intersection with more than 4 legs is not recommended. The operational effect of the presence of an intersection propagates far from the physical intersection area. For this reason, we need to consider the functional area, in which the effects of the intersection propagate.

The functional area on the approach (leg) to an intersection consists in:

1. Perception-reaction distance
2. Maneuver distance
3. Queue-storage length [coda in attesa dell’intersezione]

Intersections are not so easy to be defined as they depend on different factors. From a theoretical point of view, every leg can be calculated from a series of geometric parameters.

Main types of intersections:

• By the Italian functional classification, the intersection assumes the same rank of the highest road class that converges in it.
• Another classification is only based on geometric factors. It is internationally adopted:

- INTERSECTIONS [intersezione] are located at the same grade.

- INTERCHANGES [svincolo] have ramps and lanes with grade separation.

ANALYSIS AND DESIGN OF MANEUVERS IN INTERSECTIONS:

a) SIMPLE (BASIC) MANEUVERS: each line represents a stream of vehicles (traffic stream). Conflicts are highlighted.

b) COMPOSITE MANEUVERS: composition of basic maneuvers.

HOW TO DESIGN AN INTERSECTION:

1. Define origin and destination.
2. Establish the main streams (3-4).
3. Establish how many grades (at least, 1 or 2). The designer has to decide if the intersection can be

carried out at thesame grade (intersection), or if it needs some adding grade/s (interchange).

4) Connect origin and destination

LM Ing. Civile PoliToM. Bassani: Design of Transportation Infrastructures (a)

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HOW TO DESIGN AND A FOUR LEG INTERSECTION A total number of crossing, merging anddiverging points can be individuated. Thenumber of total conflict points is the sum of allof those points N = 32.à tot

HOW TO DESIGN AND A FOUR LEG INTERCHANGE Here, the cloverleaf [quadrifoglio] solutionprovides no crossing points and no divergingpoints. Only the merging maneuver is allowedto solve the interchange N = 16.à tot

With the same number of origins and destinations, an interchange is typically safer than an intersection, because of thedrastic reduction of the conflict points, although the construction costs are more elevated, obviously.According to the Italian Standards (Italian National Highway Code), three intersection categories are distinguished:

1)

Interchanges ramps organized on separated level where crossing maneuver is never allowed.

Intersection with grade separation ramps organized on separated level where one or more crossing points occur. This term is not used in American terminology. Intersections with grade separation are interchanges, even if some crossing points occur.

At grade intersections turning and crossing maneuvers coexist on the same area. NB: the intersection number reported has to be interpreted as a minimum value.

AT GRADE INTERSECTIONS

Their classification depends mostly on the number of intersection legs, the topography, the traffic volumes, the speeds and the desired type of operations.

Intersections with 3-4 legs generally operate easier and must be preferred to multi-leg ones. When volumes are light and stop control is used, it can be satisfactory to have all the intersection legs

departure = road area near the functional area

EXAMPLE OF THREE-LEGS RURAL INTERSECTIONS

(a, b) Tapered design type [taper = ago di manovra]

(c, d) Parallel design type

SPECIALIZED (AUXILIARY) LANES

According to the Italian Policy:

In particular, RIGHT-TURN DEDICATED LANES have to be designed and calculated according to a kinematic formula that takes into account speed and acceleration/deceleration. Islands can be used in order to control speed and deceleration.

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Respect to LEFT-TURN DEDICATED LANES, the left-turn maneuver should be allowed in a dedicated area that removes it from the through lanes. This could be provided by using islands along major arterials, although island could be avoided in case of rural roads, because of the lower speed. HCM indicated that left-turn lanes at signalized intersections should be installed as follows:

a) Where fully protected, left-turn lane has to be

stopping before colliding with other incoming vehicles within the intersection SSDàThese triangles are necessary without the presence of traffic lights. Despite this, the standards include this requirement by considering small sight triangles.

The geometry of a clear sight triangle is:

1. Such that when the driver of a vehicle without the right of way sees a vehicle that has the priority on an intersecting leg, the driver of that potentially conflicting vehicle can also see the first vehicle.
2. The sufficient sight distance for a stopped driver on a minor-road approach to depart from the intersection and enter/cross the major road. Departure sight triangles should also be provided.

RURAL DRIVEWAYS [INGRESSI] and URBAN DRIVEWAYS [PASSI CARRABILI] PRESCRIPTIONS

ROUNDABOUTS

Originally, circulatory intersections (ROTARIES) have been used for a long time in order to solve traffic issues in cities, especially in Europe. They were characterized by a large diameter (often more than 100 m) and

A long weaving area, that resulted in travel speeds within the circulatory roadway that exceeded 50 km/h. The traditional "yield-to-the-right" rule was the operating rule for traffic stream problems of congestion. Besides, high circulating speeds due to the large diameter, made maneuvers within the circle more challenging and unsafe.

In order to solve these problems, to improve safety and avoid congestion, new modern designs of roundabouts were introduced 30-40 years ago in Europe. Vehicles on the circulatory roadway of the roundabout have the right-of-way and all entering vehicles on the approaches have to wait for a gap in the circulatory roadway.