diff --git a/tests/analysis/test_label.py b/tests/analysis/test_label.py
index 50912064..fce26156 100644
--- a/tests/analysis/test_label.py
+++ b/tests/analysis/test_label.py
@@ -1,11 +1,9 @@
 import os
 
-import numpy as np
 import pandas as pd
 import pytest
 
 import trackintel as ti
-from trackintel.analysis.labelling import _check_categories
 
 
 class TestCreate_activity_flag:
@@ -100,14 +98,3 @@ def test_simple_coarse_identification_projected(self):
         assert tpls_transport_mode_3.iloc[0]["mode"] == "slow_mobility"
         assert tpls_transport_mode_3.iloc[1]["mode"] == "motorized_mobility"
         assert tpls_transport_mode_3.iloc[2]["mode"] == "fast_mobility"
-
-    def test_check_categories(self):
-        """Asserts the correct identification of valid category dictionaries."""
-        tpls_file = os.path.join("tests", "data", "triplegs_transport_mode_identification.csv")
-        tpls = ti.read_triplegs_csv(tpls_file, sep=";", index_col="id")
-        correct_dict = {2: "cat1", 7: "cat2", np.inf: "cat3"}
-
-        assert _check_categories(correct_dict)
-        with pytest.raises(ValueError):
-            incorrect_dict = {10: "cat1", 5: "cat2", np.inf: "cat3"}
-            tpls.as_triplegs.predict_transport_mode(method="simple-coarse", categories=incorrect_dict)
diff --git a/trackintel/analysis/labelling.py b/trackintel/analysis/labelling.py
index 031efdb5..cd330603 100644
--- a/trackintel/analysis/labelling.py
+++ b/trackintel/analysis/labelling.py
@@ -1,6 +1,7 @@
 import datetime
 
 import numpy as np
+import pandas as pd
 
 from trackintel.geogr import get_speed_triplegs
 
@@ -81,90 +82,39 @@ def predict_transport_mode(triplegs, method="simple-coarse", **kwargs):
         categories = kwargs.pop(
             "categories", {15 / 3.6: "slow_mobility", 100 / 3.6: "motorized_mobility", np.inf: "fast_mobility"}
         )
-
-        return _predict_transport_mode_simple_coarse(triplegs, categories)
+        triplegs = triplegs.copy()
+        triplegs["mode"] = _predict_transport_mode_simple_coarse(triplegs, categories)
+        return triplegs
     else:
         raise AttributeError(f"Method {method} not known for predicting tripleg transport modes.")
 
 
-def _predict_transport_mode_simple_coarse(triplegs_in, categories):
+def _predict_transport_mode_simple_coarse(triplegs, categories):
     """
-    Predict a transport mode out of three coarse classes.
+    Predict a transport mode based on provided categories.
 
     Implements a simple speed based heuristic (over the whole tripleg).
     As such, it is very fast, but also very simple and coarse.
 
     Parameters
     ----------
-    triplegs_in : Triplegs
+    triplegs : Triplegs
         The triplegs for the transport mode prediction.
 
     categories : dict, optional
-        The categories for the speed classification {upper_boundary:'category_name'}.
+        The categories for the speed classification {upper_boundary: 'category_name'}.
         The unit for the upper boundary is m/s.
-        The default is {15/3.6: 'slow_mobility', 100/3.6: 'motorized_mobility', np.inf: 'fast_mobility'}.
-
-    Raises
-    ------
-    ValueError
-        In case the boundaries of the categories are not in ascending order.
 
     Returns
     -------
-    triplegs : trackintel triplegs GeoDataFrame
-        the triplegs with added column mode, containing the predicted transport modes.
+    cuts : pd.Series
+        Column containing the predicted transport modes.
 
     For additional documentation, see
     :func:`trackintel.analysis.transport_mode_identification.predict_transport_mode`.
-
-    """
-    if not (_check_categories(categories)):
-        raise ValueError("the categories must be in increasing order")
-
-    triplegs = triplegs_in.copy()
-
-    def category_by_speed(speed):
-        """
-        Identify the mode based on the (overall) tripleg speed.
-
-        Parameters
-        ----------
-        speed : float
-            the speed of one tripleg
-
-        Returns
-        -------
-        str
-            the identified mode.
-        """
-        for bound in categories:
-            if speed < bound:
-                return categories[bound]
-
-    triplegs_speed = get_speed_triplegs(triplegs)
-
-    triplegs["mode"] = triplegs_speed["speed"].apply(category_by_speed)
-    return triplegs
-
-
-def _check_categories(cat):
-    """
-    Check if the keys of a dictionary are in ascending order.
-
-    Parameters
-    ----------
-    cat : disct
-        the dictionary to be checked.
-
-    Returns
-    -------
-    correct : bool
-        True if dict keys are in ascending order False otherwise.
-
     """
-    correct = True
-    bounds = list(cat.keys())
-    for i in range(len(bounds) - 1):
-        if bounds[i] >= bounds[i + 1]:
-            correct = False
-    return correct
+    categories = dict(sorted(categories.items(), key=lambda item: item[0]))
+    intervals = pd.IntervalIndex.from_breaks([-np.inf] + list(categories.keys()), closed="left")
+    speed = get_speed_triplegs(triplegs)["speed"]
+    cuts = pd.cut(speed, intervals)
+    return cuts.cat.rename_categories(categories.values())