Road Dimensioning Library

In Gemini Terrain we have set up several sets of road parameters that define the road design. This includes both horizontal and vertical curvature as well as the side areas and construction of the road. The purpose is to be able to build a road that is safe and comfortable to travel for all road users and that it is aesthetic in the terrain it passes through.

Basic Criteria for the Road

• What will the road be used for?
• for industry, residential areas or main road
• driving and/or pedestrian/cycling, public transport
• How much traffic is there on the road?
• AADT, number of lanes, heavy traffic, weight
• Speed?
• Residential area or motorway --- other

Basic criteria provide the following information:

• 1 – 2 or more lanes
• Pedestrian/cycling road or sidewalk
• Road width, shoulder
• Design vehicle
• Speed
• Gradient percentage, climbing lane
• Weight restriction, superstructure
• Cutting, filling, ditch

Elements for Safe Road

• We must be able to stop
• Reaction time
• Braking distance, friction
• No water on the road, cross slope
• We must be able to see the road ahead of us
• Clear sight forward on the carriageway, eye height and object height
• Stopping sight, meeting sight and overtaking sight
• We must stay on the road
• Superelevation in curves
• Space for the vehicle in curves, tracking/width widening

Elements for Comfort and Aesthetics

Regardless of conditions, one should drive according to the conditions. The criteria should not be such that you can drive at full speed regardless of conditions.

• Vertical acceleration
• Crest and sag
• Vertical velocity/jerk
• Superelevation, up- and down-building
• Transition directly to curve
• Expectation of driving pattern
• Nearby curves must look and feel equivalent
• No surprises
• Aesthetic design
• Blend into the terrain
• Not tiring/boring

Which Physical Laws Apply for Safe and Comfortable Driving

• Not lifting off over the hilltop
• Not being compressed at the bottom of the hill
• Not sliding out of the road in the turn
• That we manage to stop for objects on the road
• That we don't get jerks when entering curves
• That the vehicle doesn't jump when building up superelevation in curves

Formula Framework

The formulas are based on physical laws and geometric calculations. They can be found in Handbook V120. They are also described for game programs. In addition, an internal basis was created for developing Road Dimension Library.

The formulas only apply to road sections and not to intersections, i.e. acceleration/deceleration, traffic, queues and gap calculations for crossing traffic.

In the program, road classes are defined for handbook N100 and handbook for agricultural roads, but users can define their own road classes as needed as shown in the practical example below.

Practical Example: Forest Road

In this example we will describe a new user-defined road. We will set up all parameters that are included in the definition of a road class in Gemini.

Basic Criteria for the Road

• What will the road be used for?
• Harvesting timber
• Mostly driving, some pedestrians, some special vehicles
• How much traffic is there on the road?
• Very little traffic, high weight
• Speed?
• Low speed, simple road

The Basic Criteria Provide the Following Information:

• 1-lane road, used summer and winter
• Gravel road, passing places every 500 m
• Road width 3 m, shoulder 0.25m on each side
• Design vehicle is Timber truck combination 24m
• 30 km/h, no safety factor
• Gradient max. 10%, cross slope 5%, superelevation 5%
• Cross slope at curve radius over 100m, min. radius 20m
• Agricultural road as template for road body and side area

Elements for Safe Road

• Vertical acceleration
• Can use std 0.3m/s2, but trailers must not bottom out on crest, min vertical curve 200m / sag 200m
• Vertical velocity/jerk
• No clothoids, Only radii, Transition directly to curve
• Building up superelevation 20 meters before curve
• Expectation of driving pattern
• None, calm driving
• Aesthetic design
• Minimal cutting and filling

Elements for Comfort and Aesthetics

• We must be able to stop
• Reaction time: 2 sec
• Friction factor: 0.6
• Good drainage, cross slope 5%
• We must be able to see the road ahead of us
• Eye height 2.0m, object height 0.25
• Stopping sight and meeting sight, must be able to reverse to passing place
• We must stay on the road
• Superelevation in curves
• Space for the vehicle in curves, tracking/width widening

How to Create User-Defined Road Class?

In this example we will describe a new user-defined road class. We will set up all parameters that are included in the definition of a road class in Gemini.

1. Go to Gemini Terrain Settings - Common settings - Road classes
2. Select New... to create new user-defined road class
3. Enter values in the tabs as shown below

Road Class Id

• Id/Description/type
  Description of the road and categorization
• Narrow road
  Important parameter for roads with one carriageway. If this is marked the vehicle can use the entire carriageway in curves, smaller width widenings
• Vehicle
  Design vehicle for calculation of curve widenings and tracking
• AADT high/low
  Traffic volumes that describe the road type, only as information
• Requires clothoid
  Whether the curves should be built with clothoid or not: A – R – A or only R
• Fixed minimum radius
  Default value when constructing the road
• Fixed minimum vertical radius
• Crest - so that trailer coupling or drawbar doesn't bottom out
• Sag - so that the trailer doesn't touch the cab on the truck

Speed

The speed is used in calculations for what is the minimum radius in curves (together with superelevation), sight calculations

• Speed limit
  The ordinary speed limit on the road
• Speed addition
  A general safety addition
• Speed profile addition
  An extra safety factor that depends on the curve radius. Larger radius higher speed addition. The values form a curve that is interpolated in calculations (little used)

Vertical velocity and acceleration are linked to driving comfort. It has no safety significance.

• Max. vertical velocity
• Wheelbase
  These 2 values are connected, it is the vertical velocity of one wheel on the vehicle in relation to the other. Used for calculation of length for building up superelevation in curves
• Max. vertical acceleration
  Used for calculation of sag so that you don't feel that you are being pressed down into the seat

Road and Lanes

Description of the carriageway with left side, center divider and right side of the road seen forward in the carriageway, with increasing profile number.

• Shoulder
• Lanes
  Define multiple lanes. Note that for narrow roads and when you want roof fall on the road, half the lane width should be specified for right and left side.
• Inner shoulder
• Center divider
• Width
• Type
• Alternative road (info only)

You can reduce the width widening/tracking that the design vehicle has in curves. Here you also take into account whether you want the vehicle to use the shoulder when driving in curves.

• Width reduction
  If the road/lane is 3.50m and the vehicle 2.60m then you can reduce the width widening by 0.90m
• Steering addition
  Safety distance for imprecise driving in the carriageway
• Interpolation length
  The transition between the straight line and the calculated width widening based on standard calculation of tracking in curves. With asymmetric tracking the tracking calculation is followed completely out on the straight stretch

Superelevation

Here superelevation is described which is across the road and gradient which is along the road. When combining gradient and superelevation it is specified as resultant fall.

• Max. gradient
  The maximum gradient you can have on the road.
• Superelevation function
  These are values in a diagram that describes max. superelevation as a function of curve radius.
• Resultant fall
• Minimum: To ensure drainage on the road. It can still occur during building up of superelevation that resultant fall becomes 0 m/m
• Maximum: Occurs on steep sections with building up of superelevation in curves.

Stop

These are central factors for the safety calculations on the road. These factors are usually based on wet asphalt and depend on the speed.

• Friction coefficient
• Braking - used for calculation of braking distance
• Lateral - used for calculation minimum curve radius at specified superelevation so that the car doesn't slide out of the road
• Safety factor
  An extra factor to prevent accidents.
• Reaction time
  How long it takes from seeing the problem to starting to brake. This together with braking distance is stopping distance
• Eye height
  Eye height in the vehicle
• Object height
• Stopping sight - height of object you should be able to stop for
• Meeting sight - height of oncoming vehicle