Projected Length Analysis Speed Estimator

Speed from video using a known length carried by the moving object. No camera calibration, no ground reference points.

project settings

Sets the length and speed units together for every figure below. Acceleration is reported in g either way.

reference length

A fixed distance on the object measured along the direction connecting the two tracked features. Wheelbase is the usual choice. Speed estimates strongly depend on this value.

feet
Same unit as the length. Leave at 0 to treat it as exact.

Fills the true length from the Canadian Vehicle Specifications set, served by the NHTSA vPIC API. Applies only when the two tracked features are the front and rear wheel centres on one side of the vehicle.

pixel uncertainty

The width in pixels of the range within which each feature could reasonably be sitting. If the wheel contact patch could be anywhere across about three pixels, enter 3. Errors are taken as uniform across that range and independent between the front and rear points.

Judge it on the frame that reads most clearly and say which one that was. The range is carried to the other frames in proportion to the feature length, which holds the uncertainty in real units constant across the run. Choose Every frame to apply one range throughout.

tracked points

One observation per row: time, X, Y, and an optional fourth column that is read and ignored. Columns may be separated by spaces, commas, semicolons or pipes. Both datasets are matched by timestamp to determine the feature length in pixel units and the object position.

method

A rigid length carried by the object, normally the wheelbase, is measured in the image at every observation. The apparent length L' in pixels against the known true length L gives the local image scale r = L / L'. Scaled displacement over elapsed time gives speed.

Because the reference is measured in the same frame, and the same part of the image, as the displacement it scales, no camera calibration is needed and lens distortion largely divides out.

Two solvers are offered. Length scale tracking applies the local scale to the image displacement. Cross ratio uses the projective invariance of the two features seen in two frames, after Choi et al. It is exact under a projective camera but requires the object to travel along its own axis. Whichever solver is not selected is reported alongside for comparison.

Uncertainty is propagated by Monte Carlo from the selection range you enter. Speed is exactly proportional to the reference length.

Cross-ratio solver after Y. Choi, J. Park, Y. Yun, W.-J. Jeon and S.-H. Kong, "Cross-ratio and vehicle dynamics-based speed estimation for traffic accident analysis," Forensic Science International 378 (2026) 112675. doi:10.1016/j.forsciint.2025.112675

go beyond simple speed calculations

Projected Length Analysis speed estimators cover one narrow case. They need a rigid feature of known length on the object, travel that stays close to the direction of that feature, and a path straight enough that the local scale can be carried across each interval. Sideslip, post-impact rotation, a curving trajectory, or a subject with no measurable reference all fall outside them, and none of it is recoverable from a single scale factor.

Virtual CRASH 6 can also work with your scene geometry. Calibrate the camera against surveyed points and every pixel gains a position in three dimensions, so trajectory shape, rotation and motion in any direction are measured rather than assumed.

  • Camera calibration and matching. Solve camera position, orientation, field of view and radial and tangential distortion from reference points, on or off the ground, taken from point clouds, orthomosaics, scanner data or Google Maps imagery.
  • Orthorectification. Rectify a perspective video or photograph for use as an analysis underlay. Video can be dragged straight into the scene.
  • Projection into the scene. A calibrated camera projects its video or still back onto the terrain and any object set to receive projection, so footage and survey data can be compared directly.
  • Point and object tracking. Manual point tracking, automatic point tracking and automatic object tracking, each producing speed against time.
  • Track editing. Added in the Summer 2026 update. Tracks become editable polylines. Drag a control vertex in 3D and the pixel location is recomputed, or open the image plane to set the pixel and the frame time directly. Insert or delete points, merge tracks built by any method, and watch the speed graph update as you work.
  • Projection error control. Constrain tracked points to a chosen path or to a known height, which removes the scatter that comes from projecting an elevated point onto the terrain.
  • Volatility reduction. Minimum delta time, linear average and Butterworth filters, and pixel averaging across neighbouring frames.
  • Dashcam workflows. Frame by frame camera tracking for position and speed from a moving camera, and point tracking against surveyed scene features.
  • Export. Track point position against time appears in the dynamics report, which is the format this tool reads.

See Easy Video and Image Analysis for worked examples, and the Summer 2026 update for the track editing tools. Free trial.