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A data visualization web app for motor vehicle collisions in New York City
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CollisionViz is a data visualization web app for motor vehicle collisions in New York City. It is built with React.js, Redux, SASS, Google Maps JavaScript API, Google Charts, and NYPD Motor Vehicle Collisions API.



NODE_ENV=production npm install


  1. Select the date. A collision heatmap and a time distribution chart of all the collisions on that day will show up.
  2. Select the start time of the visualization by using the input box or clicking on the chart bars.
  3. Start/pause the visualization. Markers representing collisions will show up on the map. Click on markers for collision details.


For A More Intuitive User Experience

  • HTML5 inputs types (date, time, and range)
  • Disabled settings during visualization
  • Animations of the enter/leave of chart, more settings, and map options
  • Same place for the loading spinner, the start button, and the pause button
  • Toggle switches instead of default checkbox type inputs
  • The column chart shows the number of collisions in each hour, in three categories: without any injuries/deaths, with injuries but no deaths, and with deaths. Hovering on chart bars shows the statistics, and clicking on them changes the map time.

From Google Maps JavaScript API

  • Info window shows the details of a collision. Entries with '0' values are hidden.
  • Custom markers show collisions involving taxis, bicycles, motorcycles, and collisions that caused pedestrian injuries or deaths.
  • A marker clusterer with a number label will replace original markers if a few markers are too close to each other. Clicking on the clusterer will zoom the map in, and individual markers will show up.
  • Heatmap layer shows a heatmap based on all the collisions of the selected date.
  • Traffic, transit and bicycling Layers show the real-time (wall time, not map time) traffic, the public transit network, and bike paths, respectively.
  • Alternative map style is the Silver theme from Google Maps APIs Styling Wizard.


Sample Redux State

   * Need to show collisions happened between the start time and the finish time.
   * Start, finish and initialTime are in minutes, counting from the midnight.
  start: 480,         // 08:00
  finish: 490,        // 08:10
  initialTime: 480,   // 08:00
  date: '2017-03-13',
  loading: false,

When a new date is selected

  1. Control panel updates date and sets loading to be true, which makes control panel render the loading spinner.
  2. Map removes all the markers and the heatmap from the map. Then it fetches collision information from the data source using the Fetch API.
  3. Using promises, after the fetch finishes, the returned data (an array of collisions) is filtered. Collisions without time and location entries are discarded. Map builds a new heatmap and a new chart based on the filtered data. The filtered data are also reorganized and stored in an object.
// In the map component, fetchCollisions method.
this.collisions = {
     * Key: collision's time, in the form of the number of minutes from the midnight.
     * Value: an array of collisions.
    0: [
        contributing_factor_vehicle_1: 'Driver Inattention/Distraction',
        contributing_factor_vehicle_2: 'Unspecified',
        time: '0:00',
        borough: 'BROOKLYN',
        contributing_factor_vehicle_1: 'Outside Car Distraction',
        contributing_factor_vehicle_2: 'Unspecified',
    5: [
  1. Map sets loading to be false, then control panel replaces the loading spinner with the start button.

When a new time is selected

Control panel (the input box) or map (clicking on chart bars) updates start, finish, initialTime in the Redux state. Map Info and markers are updated.

When the start button is pressed

  1. Control panel uses the step function to increase start and finish in the Redux state by 1. It also handles several edge cases.
// In the control panel component
const START_TIME = 0;   // 00:00
const END_TIME = 1439;  // 23:59

step() {
  let newTime = this.props.finish + 1;
  // Edge case 1: Stop the visualization if newTime is later than 23:59.
  if (newTime > END_TIME) {
  } else {
    let start = newTime - this.state.collisionMapTime + 1;
    let finish = newTime;
    // Edge case 2: the start time should not be earlier than the initialTime.
    start = start > this.props.initialTime ? start : this.props.initialTime;
    // Edge case 3: the start time should not be earlier than 00:00.
    start = start > START_TIME ? start : START_TIME;
  1. To start the visualization, control panel uses setInterval to call step repeatedly. Correspondingly, it calls clearInterval to pause the visualization. If the user clicks on the start button again, the visualization will resume from the previous finish, when the visualization was paused.
  2. Map receives the new start and finish, and it selects collisions happened in the time range.
// In the map component, updateMarkers method
let collisionsArray = [];
for (let time = start; time <= finish; time++) {
   * Collisions:
   *   Key: collision's time, in the form of the number of minutes from the midnight.
   *   Value: an array of collisions.
   * If collisions[time] is undefined, then no collisions happened on that time.
  if (time in this.collisions) {
    collisionsArray = collisionsArray.concat(this.collisions[time]);
  1. The collisionsArray is sent to the MarkerManager, which updates the markers on the map. To facilitate this process, objects are created, which enables O(1) lookup time, and array's filter and forEach functions are used.
// In marker_manager.js
updateMarkers(collisionsArray, taxi, bike, motorcycle, ped, usingMarkerClusterer) {
  * Create an object for current collisions.
  *   Key: collision.unique_key
  *   Value: collision
  const collisionsObj = {};
    collision => {collisionsObj[collision.unique_key] = collision;});
   * Pattern:
   * array.filter(element => !(key in object))
   *  .forEach(element => create/remove marker);
   * 1. New markers are created for new collisions.
   * this.markersObj is an object with
   *   Key: collision.unique_key
   *   Value: the marker of the collision
   * Thus if collision.unique_key is not a key in this.markersObj,
   * then this collision does not have a marker, and we need to create one.
    .filter(collision => !(collision.unique_key in this.markersObj))
    .forEach(newCollision => {
      this.createMarker(newCollision, taxi, bike, motorcycle, ped, usingMarkerClusterer);
   * 2. Old markers for collisions that are not in the `collisionsArray` are removed.
   * If collisionUniqueKey is not a key in collisionsObj,
   * then this collision has a marker, and it needs to be removed.
    .filter(collisionUniqueKey => !(collisionUniqueKey in collisionsObj))
    .forEach(collisionUniqueKey => {
      this.removeMarker(this.markersObj[collisionUniqueKey], usingMarkerClusterer);
  1. About marker clusterer: A MarkerClusterer instance is created in the MarkerManager constructor, which takes an empty array of markers as an argument. MarkerManager has a toggleMarkerClusterer method, which is called when user toggle the "Marker Clusterer" switch.
// In marker_manager.js
toggleMarkerClusterer(usingMarkerClusterer) {
  if (usingMarkerClusterer) {
     * Convert current markers to marker clusterers:
     *   Put all the current markers into an array and add the array to
     *   the MarkerClusterer.
    let markersArray = Object.values(this.markersObj);
  } else {
     * Convert marker clusterers to individual markers.
     *   Clear all the markers from MarkerClusterer.
     *   Set the map of each marker to be the current map.
      .forEach( marker => marker.setMap(;

Also, createMarker and removeMarker have some extra work if usingMarkerClusterer is true.

// In marker_manager.js
createMarker(collision, taxi, bike, motorcycle, ped, usingMarkerClusterer) {
   * Create a new marker for the collision.
   * Add a `click` listener to the marker.
   * Add the marker to this.markersObj.
  if (usingMarkerClusterer) {

removeMarker(marker, usingMarkerClusterer) {
   * Delete the marker from the map and this.markersObj.
  if (usingMarkerClusterer) {
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