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Path Planning

Self-driving car path-planning on a highway.

Overview

This project is part of Udacity's Self-Driving Car Nanodegree program and most of boiler-plate code comes from the program's lecture notes and quizzes.

A self-driving car on a simulated highway road is tasked with planning its path. The controller provided by the simulator makes sure that the waypoints are reached perfectly, with a time-interval of 0.02 seconds. The following constraints exist for the ego-car to drive on this road.

  • Speed limit : 50 mph
  • Total acceleration : 10 m/s^2
  • Total jerk : 10 m/s^3

Dependencies

Build and Run

  1. Start the simulator.
  2. From the parent directory, run the following. It builds and runs the path_planning binary. 
     sh run-path-planning.sh

Directory Layout

  • src : Contains source related to behaviour-planning and path-planning. Also includes vehicle.cpp class which encapsulates a vehicle's position and kinematic properties like speed, Frenet coordinates, lane ID, etc.
  • data : Has the global map information of the highway with details such as map's waypoint coordinates.

Path Planning

Let's go through the steps that the path-planner takes to navigate around the highway maintaining its speed, acceleration, jerk and safety constraints. Please note that rest of this document refers to the source in src/ directory.

Inputs

The path-planner is given the inputs of the ego-car's localisation data and the sensor-fused data containing the information of detected vehicles.

The localisation data includes the following information of the ego-car.

  • Global position : Both Euclidean (x, y) and Frenet (s, d) coordinates.
  • Yaw
  • Speed

The sensor-fusion data includes the below information of other detected vehicles.

  • Vehicle ID
  • Velocity in x and y direction
  • Global position in Frenet coordinates

Output

As an output, the path-planner provides the simulator with a set of waypoints to achieve. These waypoints are computed using the approach described in the next section.

Methodology

The approach taken to plan a path is broken down into three main steps: Prediction, Behaviour Planning, and Trajectory Planning. Each of these steps are described in the following sections referring to their implementation in main.cpp file.

drawing

Path Planning Components. Source: Udacity Self-driving car Nanodegree

Prediction

Using the sensor-fusion input, the ego-car estimates the lane, position, and the next position of nearby vehicles. This is implemented in lines 111-138.

First we create an instance of Vehicle for each nearby vehicle detected. Their projected coordinates are then calculated based on past trajectory waypoints-size. This helps us in estimating their position while executing the current trajectory. We then determine whether the detected vehicles are or would be in the same lane, left lane or in the right lane. That leads us to decide what behaviour the ego-car should choose, as described in the next step.

Behaviour Planning

In order to drive safely and under legal constraints, the ego-car decides the next state it wants to be in given current situation and state. A simple behaviour planner determining one of three states: {Keep Lane, Lane Change Left, Lane Change Right} is implemented in lines 141-162.

The situation that this step assesses is whether there are one or more vehicles in the ego-car's lane, or are to the left/right of its current lane. Based on this information provided by the Prediction step, the behaviour-planner picks the lane it wants the ego-car to be in. For example, if we see we're fastly approaching a vehicle in our current lane, and if there are no vehicles within a safe distance in the left lane, then we decide to Lane Change Left. The safe distance is set to 30 metres which ensures our ego-car wouldn't intend to change to a lane if there is a vehicle in that lane within 30 metres (Frenet S) distance in both forward and reverse directions.

We also set a car_speed_diff variable that is used to reduce the speed of the ego-car if it approaches a relatively slow-moving vehicle ahead. This variable is further used in trajectory planning, as described in the next section, to tune the speed at which the controller reaches the individual waypoints planned.

Trajectory Planning

The last step for the path-planner is to plan the trajectory with a set of waypoints to achieve.

Based on the behaviour that the ego-car decides to achieve from the behaviour planning step, two points from the last planned trajectory and three target points each 30, 60, 90 miles away respectively from the current position of the car are used to create a spline trajectory. This is implemented in lines 167-186.

The trajectory-planner plans the waypoints in the local XY coordinate system of the ego-car. Previous trajectory's points are added to the current list for maintaining continuity, as shown in lines 104-108. In the current trajectory we plan the path using the generated spline curve for a forward distance of 30 metres, and for a total of 50 waypoints including previous path's waypoints. Each waypoint is planned taking into consideration the reference speed, car_speed_diff, the ego-car is recommended to travel at as determined by the behaviour planner, and the amount of time it can take for the controller. Once the coordinates are planned, they are converted back to global XY coordinates. This is implemented in lines 195-216.

This step wraps up the path-planner where we have the output i.e. the next set of waypoints that the controller has to achieve.