Intersection crashes present a liability challenge familiar to every auto practitioner: both drivers say they had the green light. Police reports assign fault based on witness accounts that may conflict. The physical evidence is consistent with either account. In this situation, the signal's own timing data can resolve the dispute — or reveal something neither party anticipated.
What Signal Timing Records Show
Modern traffic signal controllers maintain event logs that record phase changes , when each signal phase begins and ends, when the yellow interval starts, when the all-red clearance interval occurs, and when the next phase begins. These logs are stored in the controller's memory and can be downloaded by a traffic engineer with access to the system. The event log for the intersection at the time of the crash tells you exactly how long each phase lasted, when the lights changed, and what the intervals were.
This data can help establish which driver was in the intersection legitimately and which entered on red or stale yellow. If the north-south phase shows a 5-second all-red clearance interval that began 2 seconds before the east-west phase went green, a driver claiming to have entered on a green who was struck 3 seconds into the east-west phase is not credible. The timing data converts a swearing contest into a mathematical problem.
Obtaining the Data
Signal timing records are maintained by the agency responsible for the traffic signal system, typically the city or county department of public works, transportation, or traffic engineering. The event logs are public records in most jurisdictions and are obtainable through a public records request. The request should identify the specific intersection by its coordinates or cross streets, specify the date and time of the crash, and ask for all signal event logs, timing plans, and maintenance records for the date of the incident plus a defined period before and after.
Send the public records request immediately after the incident. Many agencies retain event logs for a limited period , 30, 60, or 90 days is common , before the data is overwritten. A request sent weeks after the crash may find that the relevant logs are gone. If the jurisdiction's retention window has not closed, a preservation letter to the agency puts it on notice that the records are evidence in pending litigation and must not be destroyed.
Controller maintenance records, also obtainable through public records requests, show when the controller was last serviced, any reported malfunctions, and any complaints about the signal's operation at the intersection. A signal that was malfunctioning in the period before the crash adds a third-party liability theory: the agency knew or should have known about the malfunction and failed to correct it.
When to Retain a Signal Engineer
A traffic signal engineer is the appropriate expert for cases where signal timing data is central to the liability theory. The engineer can download and interpret controller event logs, analyze timing plans, calculate whether the phase durations were set appropriately for the intersection's traffic characteristics, and identify any operational anomalies visible in the data. Their testimony converts raw data into an opinion that the jury can understand and rely on.
Signal engineers are also qualified to opine on intersection design , whether the sight lines were adequate, whether the posted speed limits were appropriate for the intersection's geometry, and whether the signal phasing was designed to handle the volume and direction of traffic the intersection receives. These design opinions may support a concurrent negligence theory against the municipality responsible for the intersection's design and maintenance. For additional discussion of intersection evidence in serious auto cases, see our auto accidents resources. Related coverage of verdict trends in intersection crash cases appears in our case law and settlements section.
Preserving Signal Data Before It Is Overwritten
Many traffic management systems overwrite controller event logs on rolling cycles as short as 30 to 60 days. If the case opens late, the data may already be gone. Send a preservation demand to the responsible agency immediately after the crash, naming the specific intersection, the date and time of the incident, and the controller event logs as evidence to preserve. Obtain the agency's written retention policy in the same public records request. A documented failure to preserve records after a timely preservation demand supports a spoliation argument if the logs have been deleted.