S1Q1 · Traffic Signal Action

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Function: traffic_signal(vehicle_type, signal, emergency) -> str

Core idea: Emergency ambulance check first, then branch on signal colour.

def traffic_signal(vehicle_type, signal, emergency):
    if emergency and vehicle_type == "ambulance":
        return "Proceed Carefully"
    if signal == "red":    return "Stop"
    if signal == "yellow": return "Slow Down"
    if signal == "green":  return "Cross" if vehicle_type == "pedestrian" else "Go"
    return "Invalid Signal"

Key rules: - Emergency ambulance overrides the signal entirely - "green" has two outcomes depending on vehicle_type - Any signal other than the three valid ones returns "Invalid Signal"

---

Problem Statement

Problem

Write a function traffic_signal(vehicle_type, signal, emergency) that decides what action a road user should take based on their type and the current signal.

• If emergency is True and vehicle_type is "ambulance" → return "Proceed Carefully"
• Otherwise, decide based on signal:
  • "red" → "Stop"
  • "yellow" → "Slow Down"
  • "green" → "Cross" if vehicle_type == "pedestrian", else "Go"
• Any other signal value → "Invalid Signal"

Examples:

Call

traffic_signal("car", "red", False)

Output

"Stop"

Call

traffic_signal("pedestrian", "green", False)

Output

"Cross"

Call

traffic_signal("ambulance", "red", True)

Output

"Proceed Carefully"

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Understanding the problem

The function has two decision layers:

1. Emergency override -- checked first, regardless of signal
2. Signal branch -- only reached when the override doesn't apply

A common mistake is to check the signal first and handle emergency inside the "red" branch. That breaks the case where an ambulance is on "green" during an emergency.

Order of checks matters

The emergency ambulance condition must come before the signal checks. If you nest it inside the "red" branch, traffic_signal("ambulance", "green", True) would wrongly return "Go" instead of "Proceed Carefully".

Tracing the four examples:

vehicle_type	signal	emergency	Path taken	Result
"car"	"red"	False	emergency check fails → signal == "red"	"Stop"
"bike"	"yellow"	False	emergency check fails → signal == "yellow"	"Slow Down"
"pedestrian"	"green"	False	emergency check fails → signal == "green" → pedestrian	"Cross"
"ambulance"	"red"	True	emergency == True and ambulance → override	"Proceed Carefully"

---

Solution approaches

ExplanatoryPythonicUsing lambda

def traffic_signal(vehicle_type: str, signal: str, emergency: bool) -> str:
    # Layer 1: emergency override
    if emergency and vehicle_type == "ambulance":
        return "Proceed Carefully"

    # Layer 2: normal signal logic
    if signal == "red":
        return "Stop"
    elif signal == "yellow":
        return "Slow Down"
    elif signal == "green":
        # pedestrians cross, everyone else goes
        if vehicle_type == "pedestrian":
            return "Cross"
        else:
            return "Go"
    else:
        return "Invalid Signal"

def traffic_signal(vehicle_type: str, signal: str, emergency: bool) -> str:
    if emergency and vehicle_type == "ambulance":
        return "Proceed Carefully"
    actions = {"red": "Stop", "yellow": "Slow Down"}
    if signal in actions:
        return actions[signal]
    if signal == "green":
        return "Cross" if vehicle_type == "pedestrian" else "Go"
    return "Invalid Signal"

traffic_signal = lambda vt, sig, em: (
    "Proceed Carefully" if em and vt == "ambulance" else
    "Stop" if sig == "red" else
    "Slow Down" if sig == "yellow" else
    ("Cross" if vt == "pedestrian" else "Go") if sig == "green" else
    "Invalid Signal"
)

---

Key takeaways

01

Priority ordering in conditionals

When one condition overrides all others, check it first. The emergency ambulance check must come before the signal branches, or it silently breaks for non-red signals.

02

Nested conditions vs. early returns

Using return early flattens deeply nested if/elif/else trees. Each branch exits immediately, so later branches don't need an else.

03

Dict lookup for simple mappings

{"red": "Stop", "yellow": "Slow Down"} replaces two if/elif branches. Reserve explicit if for cases that need extra logic, like the "green" branch here.