Intersections, crossroadsIntersections, crossroads

Jan Valentin

TREN TREN lecture lecture VIIVII, 2011, 2011

Fundamental terms and definitions (I)Fundamental terms and definitions (I)

at-grade intersection roads intersect or connect in the same level

grade separated interchange (split level junction) crossing roads connect mutually in different levels

Intersection = place, where roads intersect or connect in the horizontal projection and minimally two roads are joined (united) together. Intersection provides passage of vehicles (crossing) from one road to other one.

Types of intersections differ with respect to mutual vertical position.

During road design intersections have a special importance.

most sensitive and most difficult part of the road infrastructure;

usually determine the capacity of road.

Fundamental terms and definitions (II)Fundamental terms and definitions (II)

serious exposure to traffic fluency;

limitation of transport performance of connected roads;

source of dangerous accidents;

decrease of road operation efficiency.

Impacts of inappropriately designed intersection:

Fundamental terms and definitions (III)Fundamental terms and definitions (III)

Crossing = place, where roads are intersected and they are not joined (united).

Aspects of their mutual vertical position (level) is considered railway (level) crossing at-grade crossing of the road and the railway

pedestrian crossing at-grade crossing of the pedestrian way with some other road

grade separated crossing roads are intersected, but they are placed in different levels

Influence on road design and safetyInfluence on road design and safety

Intersections have special importance in road design.

Non-appropriate design of intersection set-up can influence continuality of traffic and can reduce (constrain) design (traffic) capacity of joined roads or can be resource of dangerous accidents.

At-grade crossing of roads and railways has the same importance, especially aspects of road safety and road capacity.

Usually crossings with railway are replaced by grade separated crossing on important roads.

General characteristics of intersection General characteristics of intersection

category and type of intersection, importance and functionalities of crossing roads (design speed, intensities);

number of levels;

number of streams;

range of allowed crossing travellings;

methods of traffic canalization and management;

situation of intersection within the alignment;

economic efficiency of the design;

shape of surrounding terrain;

urban characteristic of the surrounding;

methods of transposition of cyclists and pedestrians.

Specific aspects of rural intersectionsSpecific aspects of rural intersections

Design of intersections results in providing of safe and continuous traffic streams and from specific local conditions.

Following aspects are important for rural intersections:

traffic importance of crossing roads;

future intensity of running and turning streams;

categories of crossing roads and highways;

shape of territory (terrain) and placement of intersection in the horizontal and vertical alignment.

Types of intersectionsTypes of intersectionsAccording to mutual vertical location of crossing roads three main types of intersections can be defined:

grade intersection (level crossing), where two or more roads intersect or connect on the same level;

roundabouts specific type of level crossings;

grade separated intersections, where the crossing roads are connected in different levels.

Types and standards of level crossingsTypes and standards of level crossings

without right of way

with right of way

with traffic signal lights

joining intersection (fork junction)

transverse intersection

forked intersection

set-back intersection

stellar intersection (with 5 or more lanes)

roundabout

TYPES STANDARDS

Classification of intersections and crossings according to: type, standard, design parameters and degree (grade) of rectification).

Differentiation according to level of Differentiation according to level of rectification (directionrectification (direction elementary (without rectification in odd directions and without auxiliary

lanes);

extended - with turning, shifting and merging lanes;

Directed (canalized or channelized) with direction islands and horizontal traffic marking triangles.

Lay-out of individual designs is distinguished by degree of rectifying of traffic streams in the intersection:

without rectification;

with traffic island on the minor road;

with by-pass lane for a turning left;

with by-pass lane for a turning right;

with connection lane designed for higher intensities.

Joining and forked intersectionJoining and forked intersection

T-shaped (where the one road is the running one and other road starts or ends in the territory of intersection);

Y-shaped (where the road forks to two separated roads varying from the primary direction and they have the same importance too; acute angle is typical);

combined intersection (arising by aggregation of two: T or Y intersections or by T or Y intersection with transversal intersection in mutual proximity);

On-ramp

Off-ramp

channelization

TYPES

Roundabouts (I)Roundabouts (I)

Large (diameter > 40 m)

Intersection with weaving section for vehicles, which have relatively same speed and turn in a small angle, is provided. We design this type usually in places with high traffic intensity.

Small (diameter between 20 m and 40 m)

weaving of vehicles is not possible (weaving section is not designed). Orthogonal entries force the driver to slow down. It is important to design central island and traffic lane of the circular shape. Island has to block straight drive through the intersection. There is a shellring, around the island, of studied rough surface of the width (2 or 3m) to force the driver to slow down or to drive around in small radius.

Mini (diameter between 14 m and 20 m)

When its not possible to provide minimal proportions of the small roundabout, full rided central island for spacious vehicles and to force passenger cars to slow down is designed. It is appropriate especially for local distributor roads and estate roads.

TYPES

Roundabouts (II)Roundabouts (II)

MultiMulti--lane roundaboutslane roundabouts

two-lane roundabout without marking of odd traffic lanes;

multi-lane roundabout with marked traffic lanes;

spiral roundabout.

TYPES

Spiral (turbo)Spiral (turbo) roundabouts (I)roundabouts (I)

less collision points;

ordering after entering the intersection is given by the direction of travelling + physical separation of lanes in the roundabout;

excellent experience in several developed countries.

PROS

Grade separated intersection (I)Grade separated intersection (I) with crossing points, which are defined as parts working like at-grade

intersections with crossing points and the design (solution) involves only one bridge over (bonding) => on and off ramps;

with weaving points without crossing points, but weaving points are used and weaving section has to be designed and checked for traffic safety;

without weaving sections, but with turning of ramps around (without crossing points);

special and advanced grade separated intersections.

Grade separated intersection (II)Grade separated intersection (II) with crossing points

rhomboidal, one-branch, octal, delta-shaped, unconventional

with weaving points

heart-shaped, four-leaved, three-leaved, two-leaved (adjoining quadrants), annular

without weaving sections

tubular, associated, tubular two-leaved with alternate leaves

special form grade separated intersections

parted, spiral, turbined, stellar

Grade separated intersection (I)Grade separated intersection (I)

Elements of an intersectionElements of an intersection Traffic lanes

Turning and shifting lanes

electing section

slow-down section

waiting section

Merging lanes

accelerative section

manoeuvering section

lining section

Intersection branches

Minimum radii of branches

standard 12 m and for small roundabouts 9 m

Traffic islands

Traffic signal lights

Pedestrian crossing

Turning, shifting and merging lanes (I)Turning, shifting and merging lanes (I)

Turning, shifting and merging lanes (II)Turning, shifting and merging lanes (II)

Traffic streams in the intersectionTraffic streams in the intersection

Traffic stream is created by summary of all vehicles moving in the lane following each other in the same direction.

Traffic stream consists from one or several streams moving in one or several lanes approximately in the same track.

Traffic stream moves in given direction, where the main direction of vehicles on the design road is the running traffic stream.

Traffic streams splits according to kind and type of intersection in the territory of intersection (allocated limit of intersection).

continuing straight direction;

turning right in the direction of a drive;

turning left in the direction of a drive.

Traffic streams in the intersectionTraffic streams in the intersectionTraffic lanes mutually cross, merging, turning or weaving in the territory of intersection. Places, where the tracks of traffic lanes meet, are called cross (collision) points. Theoretical collision points are marked on the T and +intersection type.

Comparison of collision points on atComparison of collision points on at--grade grade intersection and roundaboutintersection and roundabout

Collision point on two lane roundaboutCollision point on two lane roundabout

CanalizationCanalization in the intersectionin the intersection

Traffic streams guidance into explicitly defined paths (trajectories) by router or separating traffic islands.

increase the clarity of intersection solutions and direction of car travelling;

decrease of collision area of the intersection.

Router traffic island

CanalizationCanalization in the intersectionin the intersection example (I)example (I)

CanalizationCanalization in the intersectionin the intersection example (II)example (II)

CanalizationCanalization in the intersectionin the intersection example example (III)(III)

Rules for design Rules for design of intersectionsof intersections (I)(I)

transport important of each road;

forecasted traffic intensity for each stream (projection for next 20 years);

distance of odd intersections;

design and solution of precedent and following intersection to provide traffic function of adjacent sections of more important road;

design unified solution of continuous section of the road to provide sequence of driving operations;

uniformity of the road section and provision to have same values (not lower) of used design elements of intersection on the main road and in the section between two mutually adjacent intersections;

usually use T at-grade intersection prior to Y at-grade intersection;

Intersection design is derived from the conditions of safe and fluent direction of traffic streams through the intersection. Following requirements should be considered always:

Rules for design Rules for design of intersectionsof intersections (II)(II)

in grade-separated intersections must always weaker and less important stream turn right from the more important stream out and connect this stream back from the right side too;

stopping sight distances have to be guaranteed;

terrain relief, buildings and other structures close to the intersection, location of the intersection in the traffic route and its homogeneity;

economic suitability of preferred solution;

trifty exploitation of the area (environmentally acceptable).

Safety aspectsSafety aspects (I)(I)

good visibility of the intersection;

perception of traffic signs and marking of turning points;

clarity of individual levels and equipment of the intersection;

understandability of projected traffic organization;

safe thoroughfare of all traffic lanes of intersection and split road ramps;

psychological certainty of the main road.

It is necessary to design an intersection with traffic and safety aspects to provide continuality of the drive and right on time decisions:

Safety aspectsSafety aspects (II)(II)

Visibility of intersection can be improved by right placement of traffic signs and emphasize duty of the yield by establishing of traffic islands on the minor road, when somebody drives to connect main road.

Clarity of intersection rises by T crossing and by appropriate vertical and horizontal design of the main road. Right rectifying and visual (or physical) design of individual traffic lane and sufficiently visible and comprehensible (understandable) marking of the shape of intersection on the orientation board close to intersection is important too.

Psychological certainty of the main road can be emphasized by the continuous guidance of the main road (include the continuity of the surface of the main road) and result traffic marking. Keeping of the total width ordering include the shoulder of the roadway in the intersection and decision on drop shaped traffic island, traffic triangles and traffic shades in the minor road is pretty important too.