<｜begin▁of▁sentence｜>### Instruction:
As an Urban Designer, let's say you are tasked with creating a plan for a new city block. You need to visualize a directed graph representing pedestrian paths within the block. Utilizing the AbstractEdgeDrawer.draw_directed_edge method in igraph, you are required to customize the edges to display arrows indicating the direction of pedestrian flow and then save the graph as an image named 'directed_graph.png'.
### Response:
import igraph as ig
from igraph import Plot
from igraph.drawing import plot

# Create a graph
edges = [(0, 1), (1, 2), (2, 3)]
g = ig.Graph(edges=edges, directed=True)

# Define the layout
layout = g.layout('kk')  # Kamada-Kawai layout

# Initialize a Plot with a specified size
plot_instance = Plot(bbox=(400, 400), background='white')

# Add the graph to the plot with a custom drawer
plot_instance.add(g, layout=layout, vertex_size=20,
                  edge_drawer=ig.drawing.cairo.edge.CairoArrowEdgeDrawer(),
                  edge_arrow_size=0.6)

# Save the plot to a file
plot_instance.save('directed_graph.png')
<|EOT|>
