Sight distance restriction on highways’ horizontal curves: insights and sensitivity analysis

European Transport Research Review

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Table 3 Examples of SSD restrictions on horizontal curves, based on AASHTO SSD design values (SSD ≤ L), Case 1

SSD restrictions	Design speed, V_d (km/h)
SSD restrictions	60	70	80	90	100	110	120
Stopping sight distance, SSD (m), AASHTO [2]	85	105	130	160	185	220	250
Calculated radius (m): desirable (HSO = 4.8 m)^*	188	287	440	667	891	1260	1628
Calculated radius (m): desirable (HSO = 3.0 m) ^*	301	459	704	1067	1426	2017	2604
Minimum radius, Rmin (m), equilibrium concept e(max) = 8 %, [2]	113	168	229	304	394	501	667
AB: Available SSD (m), HSO = 4.8 m, R = Rmin **	66	80	94	108	123	139	160
AB: Available SSD (m), HSO = 3.0 m, R = Rmin**	52	63	74	85	97	110	127

Clarifications:
SSD (m): demanded stopping sight distance based on Eq. 1. Its design values are adopted from AASHTO [2].
Calculated radius-desirable (m): horizontal radius derived by horizontal sightline offset (HSO) and SSD according to Eq. 6.
Minimum radius (Rmin), (m): minimum horizontal radius derived from equilibrium concept (Eq. 2). Its design values are adopted from AASHTO [2].
AB (m): Available SSD derived by the minimum radius (Rmin) and HSO, according to Eq. 6a.
* Calculated radius for desirable SSD: R (desirable) = SSD² / (8·HSO), for SSD ≤ L, based on Eq. 6
**Calculated AB for Rmin: \( A{B}_{Available}=\sqrt{HSO\cdot 8\cdot {R}_{\min }} \), for SSD ≤ L, based on Eq. 6a