Tangent Line Running

After a marathon, many runners look at their watch and see 42.6 or even 42.8 km. "The course was long!" they say. Usually it was not. They simply ran further than they needed to. For a pacer, those extra metres are a real problem: they cost your group time and energy, and they make your watch lie to you.

How courses are measured#

Official road courses are measured along the shortest possible route a runner could legally take. Measurers ride a calibrated bicycle about 30 cm from the kerb and cut every corner as tightly as possible. They also add a tiny safety margin (0.1%) so the course is never short.

That shortest line is called the racing line or running the tangents. On a bend, it means aiming for the inside of the corner; between two bends that turn in opposite directions, it means running straight across the road from one inside corner to the next.

Anyone who does not follow that line runs further than 42.195 km. Weaving around other runners, swinging wide on corners and crossing the road to a water station all add metres.

How much does it matter?#

Adding 0.4–0.6 km over a marathon is common for runners who do not think about their line. Here is what that means for the 4:00 group at 5:41/km (341 s):

There is a second trap. If you pace by the watch and it shows 5:41/km, but you are running a longer line than the course, you are actually slower than 5:41/km on the measured course. Run 42.6 km at a watch pace of exactly 5:41/km and you finish in about 4:02:07, not 4:00.

Watch distance versus course markers#

The kilometre markers on the course are the truth. Your watch is a tool (see your watch is just a tool).

• Check every marker. Note your time when you pass each kilometre marker and compare it to the plan or your Wristband.
• Use the lap key. Pressing lap at each marker keeps lap times based on the real course, not on GPS distance (see Garmin settings).
• Expect drift. By 30 km the watch may be several hundred metres "ahead" of the markers. That is normal; pace to the markers.

Running tangents safely with a group#

A pace group is a big, moving object. Cutting corners with 30 runners behind you takes care.

• Look ahead. Spot the next bend early and drift gradually towards its inside. No sudden lane changes.
• Announce it. "Moving left for the next turn" gives runners time to follow.
• Respect other runners. Never cut across another group or squeeze runners against barriers.
• Stay on the course. Pavements, traffic islands and closed lanes are not shortcuts.
• Accept some loss. A group cannot run a perfect line. Aim to stay close to it, not to be perfect.

Key takeaways#

• Courses are measured on the shortest legal line, so any weaving adds distance.
• Running 42.6 km instead of 42.195 km costs a 5:41/km group about 2 minutes 18 seconds.
• Kilometre markers are the truth; use the lap key and check every marker.
• Move to the inside of bends gradually and announce it so the group can follow safely.