Cambridge data projectHow many kilometres of bike lane does Cambridge have? Measuring the City’s bikeway layer with a function
The City of Cambridge publishes its Bikeway Network as open data: every piece of bike lane, signed route and trail, with its type and its length in metres. Our older kids write one function that adds up the length of any type, then call it for each type. The kilometres in this section are our arithmetic on the copy of the layer we downloaded on 22 October 2026.
Bikeway Network, City of Cambridge: our sum of SHAPE.LEN by TYPE, in kilometres (rounded to two decimals; the seven largest types shown)| Bikeway type in the file | Kilometres (our sum) | Pieces in the file |
|---|
| BIKE LANE | 62.88 | 266 |
| SIGNED ROUTE | 43.51 | 323 |
| MULTI-USE SECONDARY | 39.27 | 343 |
| PAVED SHOULDER | 35.37 | 63 |
| MULTI-USE PRIMARY | 33.92 | 164 |
| WIDE SHARED LANE | 15.93 | 126 |
| PARK | 10.74 | 145 |
The layer holds 1,541 pieces, every one marked IN SERVICE. Its map projection measures in metres, so SHAPE.LEN is a length in metres and dividing by 1,000 gives kilometres. Added together, all the pieces come to about 247.0 km by our sum; that is our figure from the raw lengths, not a City total, and the rounded rows above will not add up to it exactly. Here is the program a 12-year-old writes, with the Grade 7 sub-program in the middle:import json
with open("bikeway-network.json", encoding="utf-8") as f:
pieces = [feature["attributes"] for feature in json.load(f)["features"]]
def km_of(kind):
metres = 0
for piece in pieces:
if piece["TYPE"] == kind:
metres += piece["SHAPE.LEN"]
return round(metres / 1000, 1)
for kind in ["BIKE LANE", "SIGNED ROUTE", "MULTI-USE SECONDARY", "MULTI-USE PRIMARY"]:
print(kind, km_of(kind), "km")
Write it once
km_of takes a type name, walks through the pieces, adds the lengths that match and returns kilometres. The four calls at the bottom reuse it, exactly as the Grade 7 notes describe.
A trade-off to discuss
Each call walks the whole list again, four walks for four types. For 1,541 pieces that is instant. Children who did the Grade 6 efficiency work can say how to make it one walk, and when that would matter.
Rounding at the end
The function rounds only after adding. Rounding each piece first would throw away a little every time, and 1,541 small losses add up. Bike lane comes out as 62.9 km either way here, but the habit matters.
A row is a piece, not a path
Why 266 pieces of bike lane? A lane is split wherever something changes, at a junction, a surface or a road segment. Our check found the shortest piece is 0.03 metres long, and 71 pieces are under 10 metres; the longest is about 2,051 metres. Counting rows tells you how the City stores the map, not how many routes a child could ride. Measuring length is the honest way to compare types.
Two more questions a class can ask of the same file. Grouped by asset type, on-road bikeways come to about 157.9 km and trails to about 89.1 km in our sums. By surface, 1,253 pieces are asphalt, 171 granular and 28 wood plank, the boardwalk sort, while 47 have no surface recorded. We leave out street and trail names in class. The City says it “uses open data to improve citizen engagement and enhance the transparency and accountability of our local government.” Contains information licensed under the Open Government Licence - City of Cambridge (licence). The City of Cambridge assumes no responsibility for this page or our calculations.
