Related Clues:

• Traffic sound
• Powder holder
• Show impatience in traffic
• Game ender, perhaps
• Unicorn feature, e.g.
• When repeated, the Road Runner's cry
• Trumpet or cornet
• Gaggle sound
• Hit the horn
• It may have valves
• Gridlock sound
• Motorist's protest
• Hooter
• 'Out of my way!' indicator
• Gridlock noise