Seven common mistakes in traffic accident diagrams

1. No direction arrows

Two cars touching, no arrows: a frequent omission, and it hides how the collision unfolded. Every vehicle gets an arrow, every time.

This mistake usually happens because the person drawing already knows the story: to them, the picture is obvious. But the reader was not there. Try this test on your own diagram: could someone who knows nothing about the crash tell which car was overtaking and which was turning? If both vehicles could plausibly have been doing either, the diagram is not finished. In My Traffic Sketch the arrow tool takes seconds: click behind the vehicle, drag along its path, done.

2. Vehicles drawn where they ended up, presented as impact positions

Rest positions and impact positions are different facts. Pick one convention, label it, and note the other in the narrative if it matters.

A concrete example of why it matters (values illustrative): V1 strikes V2 in the middle of an intersection, and both vehicles roll another 8–10 m before stopping at the far corner. A diagram of the rest positions alone shows two cars parked at a corner. Which looks like a completely different, and much gentler, collision. If you draw the impact positions, say so under the sketch or in the narrative: 'positions shown at impact; final rest positions approx. 9 m northeast'. One sentence removes the ambiguity.

3. Anonymous vehicles

Rectangles with no labels, plates or descriptions force the reader to cross-reference the text for every glance at the picture. Number the vehicles and record the plates.

This gets worse fast as a case ages. Three months later, nobody remembers that 'the gray one' meant the Camry. A diagram where each vehicle carries V1/V2, a plate and a one-line description ('blue Corolla, front-left damage') stays readable forever, and the exported report prints that same table under the drawing so the identification survives photocopying in black and white.

4. Invented precision

Writing '23.4 ft' when you paced it off is worse than writing 'approx. 25 ft'. False precision is the fastest way to lose a reader's trust: and diagram measurements are illustrative until verified regardless.

The rule of thumb: record numbers at the precision of the method that produced them. A tape measure earns one decimal; a measuring wheel earns whole units; paces earn 'approx.' and nothing finer. Mixed credibility contaminates. If one distance on the sheet is visibly invented, the reader starts doubting the honest ones too.

5. No north, no scale

Without a compass the words 'northbound' in the narrative cannot be matched to the picture; without a scale reference, distances are guesses. Both take seconds to add.

These two omissions also make a diagram impossible to compare against anything else: a map, a satellite photo, another witness's sketch. With north pinned and a scale bar printed, anyone can lay your diagram next to the real intersection and check that the geometry holds. In the editor, drag the compass to match reality and pick the meters-per-square (or feet) once: the scale bar updates itself.

6. Clutter

Every parked car on the street, every tree, every building. Irrelevant elements bury the relevant ones. If it did not influence the collision, it does not belong.

The test for every element is one question: did it affect what happened, or does it help locate what happened? The parked truck that blocked V2's view passes (it affected). The building on the corner might pass (it locates). The six other parked cars fail. A useful side effect of drawing lean: the few context elements you DO include suddenly carry meaning, because the reader knows you chose them.

7. The diagram and the narrative disagree

The text says V1 turned left; the drawing shows it going straight. Contradictions between picture and words are what opposing parties look for. Draw first, then write the narrative while looking at the diagram: or use a tool that exports them on the same page so the mismatch is easy to catch.

The most common source of this mistake is editing one and forgetting the other: the drawing gets corrected after a conversation with a witness, and the narrative keeps the old version. Before exporting, read the narrative once while tracing each sentence on the picture with your finger. Every claim in the text should have a visible counterpart. An arrow, a position, a mark. If a sentence has nothing to point at, either the drawing is missing something or the sentence is.

The 60-second review that catches all seven

Before you export, run the finished diagram through this quick pass:

• ▸Cover the narrative: can a stranger tell who moved where? (catches 1)
• ▸Ask 'is this impact or rest?' — and check that the sheet says which. (catches 2)
• ▸Point at each vehicle and say its plate out loud. (catches 3)
• ▸For each number, name the tool that measured it. (catches 4)
• ▸Find north and the scale bar without hunting. (catches 5)
• ▸Justify every element in one sentence: delete what you can't. (catches 6)
• ▸Trace the narrative on the picture, sentence by sentence. (catches 7)