a-b-street/abstreet · error

It's now, so you can't start a trip at

Error message

It's {} now, so you can't start a trip at {}

What it means

The /sim/new-person command validates that every trip's departure time is not before the current simulation time; the simulation can't spawn a trip in the past. It rejects the whole request if any trip departs before sim.time().

Solutions

1. Clamp or shift trip departure times to be >= the current /sim/get-time value before submitting.
2. Reset the simulation (/sim/reset) so time returns to 0 and original departures are valid again.
3. Ensure your scenario generator and the sim use the same time units/format for departure.

Example fix

// before
await post("/sim/new-person", person); // departures from t=0 schedule
// after
const now = parseTime(await post("/sim/get-time"));
person.trips.forEach(t => { if (t.departure < now) t.departure = now; });
await post("/sim/new-person", person);

const now = parseTime(await post("/sim/get-time"));
if (person.trips.some(t => parseTime(t.departure) < now)) throw new Error("trip departure is in the past");

try {
  await post("/sim/new-person", person);
} catch (e) {
  if (String(e).includes("can't start a trip at")) {
    const now = await post("/sim/get-time");
    person.trips.forEach(t => { if (t.departure < now) t.departure = now; });
    await post("/sim/new-person", person);
  }
}

When it happens

Trigger: POST /sim/new-person with an ExternalPerson JSON body where at least one trip.departure < sim.time() (e.g. after the simulation has already advanced).

Common situations: Replaying trip schedules recorded at time 0 after the sim has run for a while; hardcoded departure times in test scripts; clock drift between scenario generator and sim time (times in different formats/offsets).

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• is in the past. call /sim/reset first?
• Bad TimeInterval ..
• Bad DistanceInterval
• Can't spawn at ; it isn't that long
• Can't start at ; it's the edge of a border already

AI-assisted analysis of a-b-street/abstreet@0964f29315 (2026-09-13). Data as JSON: /api/errors/30d6dac67c5430c4. Report an issue: GitHub.

Appendix: source

Thrown at headless/src/main.rs:197

            *sim = Sim::new(&map, SimOptions::default());
            Ok("map changed, blank simulation".to_string())
        }
        "/sim/get-time" => Ok(sim.time().to_string()),
        "/sim/goto-time" => {
            let t = Time::parse(get("t")?)?;
            if t <= sim.time() {
                bail!("{} is in the past. call /sim/reset first?", t)
            } else {
                let dt = t - sim.time();
                sim.timed_step(map, dt, &mut None, &mut Timer::new("goto-time"));
                Ok(format!("it's now {}", t))
            }
        }
        "/sim/new-person" => {
            let input: ExternalPerson = abstutil::from_json(body)?;
            for trip in &input.trips {
                if trip.departure < sim.time() {
                    bail!(
                        "It's {} now, so you can't start a trip at {}",
                        sim.time(),
                        trip.departure
                    )
                }
            }

            let mut scenario = Scenario::empty(map, "one-shot");
            scenario.people = ExternalPerson::import(map, vec![input], false)?;
            let mut rng = XorShiftRng::seed_from_u64(load.rng_seed);
            sim.instantiate(&scenario, map, &mut rng, &mut Timer::throwaway());
            Ok(format!(
                "{} created",
                sim.get_all_people().last().unwrap().id
            ))
        }
        // Traffic signals
        "/traffic-signals/get" => {

View on GitHub (pinned to 0964f29315)